I am puzzled by a quick comparison of these two ultimates. I'm looking at this from the perspective of my bow-specced Khajit NB with 54 points in stamina and 10 in health (just over 30k buffed stamina ). What mystifies me is why anybody would bother with Rapid Fire compared to Soul Strike.
On these stamina-based bow users, Rapid Fire's tooltip says that it deals about 39820 damage over 4.1 seconds, for a cost of 175 ultimate. Soul Strike deals over 49364 damage over 3.9 seconds for 100 ultimate. Now, I'm no math whiz, but I really can't see why anybody would bother with Rapid Fire. Less damage over a longer channel duration for a higher cost. Yes you can move at full speed with Rapid Fire, but Soul Strike includes a 70% snare so to me that's a wash. Even the battle of the morphs doesn't help Rapid Strike, in my opinion.
Even worse, there seems to be a bug in that Rapid Fire damage is NOT affected by Relentless Focus and the Minor Savagery buff, while Soul Strike is. That brings Soul Strike up to 53416 -- more than 34% higher damage for less cost than Rapid Fire.
So first of all, fix the bug with relentless focus not affecting Rapid Fire (and look at other weapon ultimates and buffs of course). Second...why would I use Rapid Fire anyway for other than Role Playing purposes?