Funnily enough their equipment isn't actually that different on a mass scale, although the 40k era has a few small advantages and one big one. Bolters have a few more shots in 40k, Volkites are so rare as to be non-existant by the end of the GC so they're hardly a issue, and heavy weapons like plasma, melta and yes even grav are all about as common in 40k and 30k, just on a different scale.
The big advantage is the armour, Mk.7 Aquila, which is so much better than Mk.3 Crusade its not even funny. Bolters can pen Mk.3 in the limbs straight out, and a dead-on shot will punch through the chestplate as well, whereas Mk.7 is effectively immune to bolter shots across its mass. Sure joints will still be penned, but it takes Terminator armour for that not to be true. And hell, Heavy Bolters will sometimes not pen the chestplate of a suit of Mk.7, whereas it'll go in one side of a Mk.3 and out the other. That big raised Aquila on the chestplate of Mk.7 isn't just fancy, its also extra armour over the vitals. But yes, Comptemptor Dreads are miles better than Box Dreads, and I won't deny that, but they were pretty rare by the end of the GC themselves.
As for the degeneration thing, eh, it depends. Some chapters, like most Ultramarine/White Scars, are nearly identical to their forebears, while others, just about every Blood Angel, are wildly different. But they haven't degenerated as such, more moved specializations. 30k Blood Angel geneseed created the Revenant Legion for a reason, literally anyone could take it including women depending on the writer, but overall produced inferior Astartes who degenerated into maddened mutants after a few decades. This legacy of mutants and madmen incidentally is what caused the Red Thirst in the first place, and allowed the Black Rage to be formed with their Primarchs death. By 40k however Blood Angel geneseed mutated into the most Psyker heavy geneseed of the loyalists, and produces Astartes naturally inclined to flying and with oversized fast-twitch muscles in their arms. So yes they are different, but not necessarily degenenrated in such a pure sense.
But oh yeah there has to be a distinction made between traitor marine #3,473,483 and Kravaks The Bloodthirsty, survivor of the Siege of Terra and Captain of the 48th Grand Company, because the later is going to school most 40k era marines with ease. And yeah, a Named Survivor of those times is going to be a world-shattering level of broken, and someone the Loyalists don't really have a answer for anymore. Because there just isn't anyone who has undergone a crucible like the GC and survived everything. Maybe Dante due to sheer age but even thats a stretch given how long he spent doing bureaucracy and so on.
EDIT: I would probably pick 40k era marines in nearly every engagement. They are simply to good to lose most scinarios. I would even say they win at 5-1 odds 8/10, just because their bolters can penetrate the armour of the Mk.3, while the GC era marines are going to have to aim for small joints to do so, or use the unstable Flux ammo to match the 40k marines in just being able to spray and pray. Anything more than 5-1 though and I got to give it to the GC era marines 6/10, numbers would wear them down eventually. And really thats the strength of the GC era marines, the ability to drown a otherwise superior enemy in oceans of post-human soldiers who are just good enough to hurt your much more expensive elites, while also being easily able to slaughter chaff in waves.
Yep, that's why I said that there were some cases where the tech was more advanced. It's weird honestly, where you have a setting that's all about stuff from the past being superior, or at least better well known then the future, and then you've got suits of Armour that is superior to everything to the past. And this wasn't even 8th edition either, this stuff was as early as 4th edition.
I wasn't referring to just the Blood Angels. Their geneseed, at least on everything but their flaws were the same then as now. The difference in terms of inferiority, Just the widespread mess of the thirst and rage. Before the end of the Crusade, only like a handful of Marines ever fell victims to the thirst, and they were easily kept a secret. NOW? In the past 2 centuries (in modern 40k), the Blood Angels were nearly wiped out on 2 separate occasions. One was because their were more Marines falling to the twin curses then ever before, which almost led to their extinction and to Dante Asking his successor chapters to give some of their own scouts, marines and even geneseed for the Blood Angels, to stop them from falling apart.
The second time, was when they almost all got consumed thanks to the Black Rage, when Leviathan trying to pick their world clean and Kabanda showed up to say, "Nah Uh!", which caused several chapters to almost completely fall to the black Rage as soon as they saw the demon.
And it's not even the Blood Angels, the Fists and the Ravenguard, have lost several organs that should be completely useable and fine for them to use. And these are the first founding chapters. It's gotten to the point that the Ravenguard ask for more of their Older and more pure Geneseed from Mars than ever before, which only adds to their ever low numbers. And don't even get me started on the Space Wolfs and their Wulfen Curse, or how now they can only recruit from Fenris and nowhere else, despite everything in 30k prior to Leman, and even parts where he does show up contradicting that.
And theirs all of the other actual chapters besides the og ones that have their own set of problems. Or problems that show up, not as geneseed flaws, but Mental ones. Where the White Scars can enter battle list trances when their Blood gets high and there are people to kill. Or the Dark Angels and their complete and total focus on distrusting others and Paranoia that is an in-tabletop effect. And lastly, the Iron hands and their full self-hatred of the Universe and the Flesh they inhabit.
That was what I meant by degenerated. Either they have drifted so far apart from their beginning thanks to in built flaws that are just getting worse and worse over time, or organs are just no longer working and the geneseed just isn't making a good enough difference. Or, because of how a set of cultural ideas have been settled into the mind of a newborn Marine, it can act and become like a flaw, such as the Dark Angels or the Iron Hands.
And then theirs the differences that exist from successor chapters from their og ones, like the Excoriators having the "Darkness", which the mainstay Fists do not have, but the Excoriators do have. The genetics for Mainstay 40k marines, by comparison to their forbears have completely fallen off the rails.
Besides that, yeah. 30k Marines just have superior ease of actually getting stuff produced and made for them, and not have it be a relic that just needs one unfortunate accident, and boom, a priceless relic that can never be remade, is now lost forever.