tbh, scratch the Galaxy TAB. it's just not fit to be a phone.
if your use is more multimedia heavy (HD videos, video calling, hi-def music), i would suggest the Galaxy S. True, it's cpu power pales in comparison to the Desire HD due to the fact the hummingbird is not optimized for andriod, the performance is pretty good and the Galaxy S GPU and sound processing murders the Desire HD. Off the bat, you can play mkv, avi, divx and xvid up to 1080p resolution w/out the need for 3rd party apps (and the Galaxy has TV-Out too). As for music, the music player has advance equalizer setting, including 5.1 simulation). And the screen - the screen is to die for and only comparable to the iphone4's retina display. Also, it comes with built in 8/16 Gig of internal storage on top of the 2GB ROM and 512 MB RAM
If you're into modding, flashing, app-intensive usage, then the Desire HD would be a better idea. With so many dev's supporting it, your options are endless. Specs on the HD say that it does support multiple video formats, but I'm not sure how broad the coverage is. Music-wise, there are a lot of music apps you can install to make up for what HTC has such as winamp and poweramp. Also, the it has an LED flash and a slightly bigger screen. Has 1.5 GB of ROM, and 768 MB RAM.
Myself, I have the Galaxy S, still running stock rom with a couple of tweaks to improve performance and I am extremely happy with it.
for specs comparison, check this out:
click me
hope this helps