Hi, I had a 31se in my system while my 50se was getting upgraded to a 51se. The 31SE hand down beat the 50se. The 51SE is similar to what flyski says above, however, I'll take it a few steps farther... I think it is deeper and more extended in the bass, by a little larger margin. More detail across the whole spectrum. Lower noise floor, better staging-wider and deeper. More natural flow to the music. The top end really shines. Goes on what seems like forever. More weight-wetness in the mids and the top end. Better detail, but more natural. Finally, dynamics are better. More pop to things. In the top end for instance, when a stick hits a cymbal...you more disincly hear the impact...then the decay...
All around better. I think it's well worth the extra $$'s. But I do agree it's system dependent. If you have the system now or plan to then you really should go for it.
My Wilson Watt Puppy's are very revealing and I definitely hear the improvement. Especially on hi rez sources like vinyl and SACD.
If you primarily listen to standard cd...maybe these differences are not as pronounced.
With all this, I still think balance is important (unless you're planning for the future). The 51SE can't be any better than what's input into it and the amp has to handle what the 51SE is sending it...ditto the speakers...
Sorry, but your checkbook is the only one that might say something. Go for the 51SE.