You will likely be able to get more spl output on the Gems. They will also likely dig a little deeper, but that isn't really a concern on surrounds.
How large is your room?
If it is on the small to medium end, then the Promonitor will likely suffice.
If it's on the medium to large end, then I would consider the Gems.
If you could find a dealer that will let you take home both and return 1, then you could see for yourself.
Eagle has a good point too!