What sort of all mountain riding are you doing? All Mountain is a little vague...
Basically, what I would say is if you see yourself going balls to the wall charging down the mountain in most cases, you'll probably want to lean more towards the SPI. Otherwise, I think the Rome390 Boss will be better for an all-around performance (especially if you plan ongoing into the park).
I like Ride Bindings, but I think in this case the alternative makes more sense.
You might want to consider some bindings in between the two you mentioned as the SPi's are consderably stiffer than the 390s. Have you thought of Rome Targas or Ride Delta Mvmts? Both these are stiffer than the 390s and softer than the SPi's and would allow you to move from moutain to park easily.
I've got a NS SL-R 161 with SPI's. I never go in the park, but like hard carves, glades and moguls. Personally, I like the instant response I get from the SPI's, particularly in moguls and tight trees. That said, if you're doing more than a random lap thru the park, you'll probably want something a bit flexier.