The X10 will be an amazing phone, do not worry. Sony Ericsson devices always slow somewhat pre-release, but when they were retail or after a few updates the performance is usually higher indeed. And do not have to use user interface and SE also the same, the robot default is also the present.
I would go for the x10. The HD2 is almost outdated because windows phone 7 will be coming out this year. It's a great looking device and is the best windows mobile 6.5 phone you can get. But the future is android. If the HD2 could get win7 then MAYBE you could consider getting it.... but it's not. Stick with android as you won't be disappointed.
