There is the passions. You get to roll or pick, and each gives a small stat bonus and penalty. This is particularly good for those characteristics whose primary purpose is their bonus, like toughness, as often enough a +3 here or there either gets it into the next tens digit, or makes it one purchase cheaper to do so with xp.
BC Legionaires are a bit weaker indeed than the Deathwatch marines, but given the XP system, they'll end up much better overall eventually. They also have a more open way of building. Of course, as usual, there's less potential for the base stats, as four upgrades with a lower start just won't get as high. However, for the same XP, you'll be way ahead in skills and talents known. Plus, infamy points once you have enough corruption are like having twice the stock of fate.
In theory, a starting CSM in BC is around 7,000 xp while a starting Deathwatch SM is at either 13,000 or 14,000 xp (I don't recall which). If you give the CSM the extra xp, he'll be individually much better, but there's no equivalent to the Squad Mode from Deathwatch. However, I really don't think that the games mesh well at all, so I wouldn't recommend ever trying something like this.