Yes, the i7 is one heck of a chip, it also carries one hell of a premium price tag as well with needing to have a new MB, expensive ram, etc. HT as you said, only can show any gain if the core's are not under full load. Under full load performance may actually slightly decrease.i7 is definitely far ahead. Nothing else to say about that.
As for multimedia, I'm still suspicious of the compiler technology; architecturally K10's SSE support looks really good on paper. It seems to me that we just haven't seen code that was generated properly to take full advantage of it. And HT really doesn't buy anything if you can keep your active cores sufficiently fed that they have fewer stalls.
Now onto SSE. Intels SSE does have a advantage in it's hardware. The intels cores have 3 128-bit units with 2 of them being symmetric vs the old K8's 2 64-bit units. With Phenom SSE performance did see an increase though with them now finally being able to execute two 128-bit generic SSE ops and 1 SSE MOV per cycle. Still the intels do carry a bit of an advantage here with it's 3 128-bit units.
Compilers do make a difference as well but greater gains can actually be had by optimizing the code for a specific processor that compilers may try to do but don't always succeed at. GMP for example when optimized with AMD specific patches abolutely will trounce on a intel with simular hand optimization (to be fair though how the i7 would respond isn't really known yet). Matlab, SuperPi, Prime95 is what intel guys love to show off for intel's "mathematical superiority" but all of those carry very intel specific optimizations or genaric optimizations without the same level of optimization done for the AMD specific CPU's.