@Kipeci In my opinion, and I will admit that is partially informed by real history information about this period, the Congress of Gela will end in a common peace modeled on the status-quo between the Sikeliotes and Syrakousai. In which everybody (so including Gela and Akragas) promises to unite against future aggressors. Once that common Sicilian peace is in place we will be free to direct our attention elsewhere. I would try to emulate such an result in this time line, especially since Syracuse is weaker here then it was in reality.
To make sure we get the terms we want at the Congress we must make an effort to appear as strong as possible. Here it is IMO Krotone that will help us the most. You bring up that Syracuse won't be able to ally Krotone, due to the power of Thurii and Eretria bearing down on them. This might be true, but let me go one step further: With the power of Thurii, Eretria AND Krotone bearing down on Lokri, it will not want to be the speed bump on our march to Sicily.
IF we manage to flip Krotone, everything you said about Lokri & Krotone is even more true for an isolated Lokri. This is what makes Krotone so attractive to me. Allying them means still retaining the temporary Rhegion alliance against Syracuse AND making it quite likely that Lokri will back down without a fight. This denies Syracuse their Italiote "distraction" and speeding up the rate at which we can reach Sicily.
In Short:
Krotone:
Short-term: More troops committed, less enemies in the upcoming stand-off with Syrakousai
Long-term: Aid against the Bruttii & Lucanians, securing our western flank against future Italiote coalitions.
Rhegion:
Short-term: No new troops/ships committed (already opposing Syrakousai), potentially worse relationship with Sikeliotes
Long-term: "Naval dominance", restraining Sikeliote ambitions, threat in the back of future Italiote coalitions.
I actually see the long-term benefits as roughly equal, with Rhegion maybe even edging out a small win. However, the short-term benefits of Krotone's aid are simply massively superior.
As to your last argument; that the alliance with Krotone will bring with it a new distraction in the form of the Bruttii, which Syrakousai will abuse to strike when we are otherwise occupied. You yourself said, that the end goal is to have the Sicilians keep down Syracuse by themselves. In that I completely agree with you. So, I'm confused then why you worry about us turning our attention to prepare for other serious threats (such as the Oscans). They alliance with Krotone doesn't mean we start warring with the Bruttii immediately. It only means, that should we in the decade to come choose to help Thurii in their struggles, we now have addtional 3,500 hoplites in the field.
In general, I would say both choices have their advantages and Rhegion would have been my clear second choice.