I upgraded from the Desire to the Desire HD and I like it. The desire HD is a bit bigger in the hand so it takes a bit of reach to get to the top. However, everything feels a bit bigger. I like the size of it, so I'm really happy.
However, they don't seem to have found a way to root it easily, ,which is somewhat limiting. With my Desire, I rooted it and put on Defrost, which as a really nice ROM which was quite speedy. The Desire HD is alot speedier, has better specs (except for the screen).
Any how, I'd go for what feels best in the hand. The Desire is becoming dated and seeing as you are upgrading from the X10 (a model that came out around the time the Desire did), you'll probably want the "better" one. I don't like slide out keyboards because of the bulk, so I can't give you unbiased information on that.
Tim