I recently upgraded from a 350exr6 to an Ultra double 550.

The difference I hear is in the mid and upper end and it is very slight / subtle. The vocals seem a little clearer to me, where there used to be songs that some of the lyrics sounded garbled, that feeling is now gone, so far anyway.
I haven't listened to all my collection just yet.
Overall the difference is minimal at normal listening levels.
I tend to listen to very loud music and movies, that's where the bigger amp with more wattage comes in. Keep in mid your speaker efficiency
Bottom line; it is a matter of $$ and volume and which is more important to you.
Best of luck,
G.