In my opinion, Princess has very comfortable beds. And the ones in the balcony cabin versus the junior suite will be exactly the same - absolutely no difference whatsoever. The only difference between the two cabins is the size - the junior suite is our favorite because it affords us more room - but that's it.
Now, should you ever find a bed on any ship to not be to your liking, simply ask your cabin steward to provide you with a 'topper'. Sometimes this will be a simple egg-crate type foam rubber topper, but other times it can be a memory foam-type topper, which we actually got on an NCL cruise a couple of years ago.
Personally, we've always loved the beds on Celebrity, Holland America, and Princess, but more and more, cruise lines are realizing how important it is to provide good bedding to their passengers, so things they are a changing.