What do you view as the advantages of both? The disadvantages? Which do you think feels right for you? Do you prefer Android or the iPhone OS? It really comes down to your decision.
I am a bit biased toward the X10 but that is because it's something different. Everyone and their dog seems to have an iPhone and I personally don't see what the big stir about it is. I don't see it as being anything innovative or anything. I mean, sure it can do some neat things but so can other phones. It's just a matter for you to decide what features and functionality you want to have and which of the two has more of those.
You already have a X10 and you obviously picked that over the 3GS and the only difference between that and the 4 is the front-facing camera and HD video capture (which the X10 will get).
So, to me, it seems like you've already chosen the X10 but like I said, evaluate what you want out of your phone and go with the one that suits your needs best. Hope this helps!