A battlecruiser means both a lot more firepower, opening up military options, and a lot more cargo space, opening up what they can transport (the Dauntless was limited to more high price-per-volume stuff). They're a lot more expensive to maintain, so they can't really afford to spend much time not earning, but they can pretty much roll up on worlds and take what they want. You can forge a new trade route very easy as you can both secure it and do a few major loads. They're very powerful.
An escort means a lot less power. Now the Trader needs to be a lot more careful about where they go- they're a lot more likely to encounter local forces they can't fight than even the Dauntless, and trading is even more limited as they have even less cargo space. However, the lower maintenance cost on the ship and it's lower profile means they don't need to worry about bringing a score back each run as long as it evens out in the long run. They're more suited to following up rumors of the really big score, aiming for lost artifacts and so on, or at least carrying the high-quality stuff (loads of lasguns aren't going to bring in the bucks at this scale. Loads of high-quality bolters, maybe!), and exploration is probably the name of the game. Find something valuable and sell it/sell the info.
Bottom of the barrel in RT power if you're a newbie with a small ship, though even that much makes you a valuable ally to some and you are in a position to gain more power and reputation, but it requires more cleverness.