Both classes are amazing, and we do need to wait to see if the 2nd subclasses will play out the way we think they will. But from what we do know, the titan can take more damage from the start from larger base armor, and excels at closer, single target and aoe damage, while the warlock recovers better to start, and has more ranged aoe that can be used faster (due to energy drain). Of course the sunsinger and defender are complete changes to this, which add so much more to the game. So with what we know, it's down to which of those two styles matches to your playstyle. Personally, I play as a warlock due to the range and recovery (and I love being a space witch) but I have huge respect for titans, and would gladly have one to support me in my fireteam any day~!