i hate to admit that i have both.
the architecture is the same.
the keyboard has the ribbon; which the desktop does not have. probably the biggest difference. and one that i don’t use very often.
obviously, the keyboard has pitch and mod wheels. i do *not* use those the way most people do. i disable the normal functions and use them for modulation sources. imo, this really helps the sound design aspect of things. for example, i have a patch that’s pretty gnarly that uses the pitch wheel to change the panning. i was using it on a piece of music and decided that i needed the ability to change the attack so it wasn’t so harsh, so i used the mod wheel as the mod source for the envelope attack.
now, if you have something like a keystep or other controller, you could use that on the desktop. you could also use the macro knobs to do it. (the macro knobs are very cool.)
this thing had a ton of ways to modulate/mangle sounds.
the pads on the desktop have poly aftertouch, so you do *not* lose that functionality. however, the range is smaller than the four octaves of the keys version (maybe 1.5-2.0 octaves?), but you can define how the pads are set up for scales, etc.
for me the thing about the hs is the way it responds to the way you play it. i know some folks are using it for sequencing, but that’s not something i do.
why i love having both: this track is made up of about three tracks that have both hydrasynths going. each patch is made of both synths using the same basic programming with slightly different lfo speeds, envelope lengths, panning, mod values, etc.
headphones are useful.