You *can* use the rest if you don't use both embellishments.
That sort of choice will always be there, since rests use up slots that could be used by other things. (We get more effective rests later on, too, though of course there's never enough rest-power to suck up all the spell-power we'd
like to use.)
In terms of having ticking songs going down, the question is interesting though. I had a quick look at VIT earlier but with newbie stats it's kind of hard to tell what it's doing and what has more of an effect. An effect from not a lot compared to a bit more than not a lot is... not a lot.
Personally I'd rather have the regen rate over the flat mana, at least with a bard - but now that fights are longer, that might be a bad strategy. I did notice that my baby shaman was OOM
all the time earlier today when I grouped - I hadn't played one in a while, and they changed costs & things recently... and suddenly mana (sorry, energy?) management was a real issue. Much more interesting in the long run, though.
So... still no answer for you, but it's going to be fun finding out.