EXR-5 is a new arranger from Roland. It will be at stores in two weeks here in Europe (I don't know in USA).
It will have a beautiful piano voice, and good keys for piano playing (Yamaha have smaller keys and with a bad feel).
About tweeter, I don't know... But PSR550 doesn't have tweeters, I think.
Perhaps other Yamaha kbs have better piano voice and better keys (DGX series, for example, or PK1) and have good price.
But you must see, play and listen before buy.
Cheers