I personally couldn't get into Warhawk and that's why I'm not hyped for Starhawk. But I can tell you that Starhawk looks far more interesting than Warhawk and that's at least a start for me. I wanna play a demo or beta of Starhawk before I decide whether to get hyped for this or not.
On the other hand, I never played Twisted Metal before but I am definitely keeping my eyes on TM because of the Split Screen gameplay. Split Screen gaming with friends online is just too much fun and I know my friends will love to play this and my brother is also hyped for TM so for me Twisted Metal is looking good.
So regards to questions:
1) I am more interested in Twisted Metal and will most probably buy it. Starhawk I cannot say the same for as of right now.
2) Twisted Metal will sell pretty good if word of mouth gets out there. Though I think Starhawk could sell more.
3) Again this will be tight. I see fun and quality coming from both games so this one I cannot determine but metacritic wise, both will be in the same range and very minimal score difference. ie if Twisted Metal is around 88, Starhawk could be at 90, 86
