I would suggest looking at the rest of your group.

Are there a lot of melee's? If yes, it might be hard to proc BB's secondary damage, so it might be better to stick with twf (I dont like the dual wielder feat though, I would stick with shortswords or scimitars). If not, then BB might be worth investing in (mc;ing to get it, or through variant halfelf or elf, or through magic initiate).

Also, do you have a reliable enough way to get advantage (such as sth granted ny an ally or a familiar, etc)? The BB is probably better than twf, since the advantage probably plays better with BB rather than with twf (although I have not run the numbers against some expected AC value), but this again brings you back into the point in the previous paragraph (ie, how often do you think you will be able to proc BB's secondary damage?).

----------------------------
Multiclassing in warlock has some sweet synergy, but there is no right answer as to whether you should multiclass or not (at least as far as I can think of). Depends on what you really want your character to do. The easiest thing to do in order to decide is to have a look at some warlock spells and perhaps even invocations, and then ask yourself: ''Do I really wanna have mu character do that?''.

And since multiclassing comes at a serious cost, you better make sure you wanted sth badly out of warlock.