Levels of interest/how long ago I played it the last time.
To me, I usually buy games that I have seen enough discussion and reviews thereof around it that I know it's going to be good, what determines if I will play it is how fresh it is to me.
So, let's say I have been playing Shogun 2 for a long time, but not so much Fallout New Vegas, I will go for Vegas because it seems more interesting and fresh.
That's about it really.
To me, I usually buy games that I have seen enough discussion and reviews thereof around it that I know it's going to be good, what determines if I will play it is how fresh it is to me.
So, let's say I have been playing Shogun 2 for a long time, but not so much Fallout New Vegas, I will go for Vegas because it seems more interesting and fresh.
That's about it really.