Video seems smoother on the samsung too.
I gather you mean visually smoother due to the super amoled?
Personally I was in two minds about the Galaxy S before the DHD came along, it's a little plasticy and I'm not a fan of the design, it is lighter and thinner than the DHD albeit by a small amount and of course has the super amoled screen with better colours and blacks.
The desire HD is built very sturdily with the aluminium body it feels great, the screen size is definitely a great bonus too and the SLCD is good for browsing and general use.
So physically you've got a slightly bigger and heavier phone with the DHD but the screen is bigger and it feels so much nicer to hold. DHD also has a lot of extra sense features built in.
Both devices now have 2.2 and with it quadrant benchmarks them both similarly overall but the benchmark breakdown is quite telling, different strengths on both devices:
Both have excellent onboard space for apps too, I don't think you'll ever run out. Personally I think with 2.2 both devices are very powerful and capable, I think the main thing that will make you choose is to hold them both in your hand and get a feel for how they are in the hand and how you like one or the other screen too.
Personally I picked the Desire HD, all round I think it's more powerful in the right places, I love the aluminium case and overall design, not to mention the huge screen is great.