mkirsch wrote:
Well, if the 55W "can't quite" keep up, and you're more than doubling your solar capacity, I would hazard a guess that you are good to go.
Worst case you can add the 55W panel back in by changing your solar controller.
You've got nothing to lose, and everything to gain, by trying with the 120W panel. You can't add the 55W or any other panel in without spending money on a new controller anyway. If you don't need it, you're money ahead.
+1
And a controller capable of just a 55W panel would be very inexpensive.  
Here's a link to one from a reputable brand for $33.  I'd get one and hook both panels and controllers up to the batteries.