I have personally owned an htc Diamond 2, but I didn't like it very much. Maybe the reason was that I don't like Windows Mobile. So I would suggest that if you are not used to Win Mob, don't buy it.
I have used XPERIA X10 and I would say, IT IS AWESOME! You will LOVE that phone. The camera is AMAZING.Go for it.
I haven't used Acer neoTouch so can't recommend you that.
Best of Luck.