I started out with the Yamaha PSR line, but I found I was never using the "styles" or any of the other "special features" of those boards; and I too didn't see the point of the built-in speakers. Frankly, as the built-in speakers go I think the PSR-262 (which you can get for under $200) sounded much better than the speakers on the PSR-550 (which you'll be paying more in the arena of $500 for.)
Where I landed was the Motif6. It doesn't sound like this is the board for you though -- it is about 3 times as heavy as the PSR-550, and you mentioned weight as a factor. I have to agree with the Jocko and say that you should not worry about the speakers -- go for the board that matches your personal critera minus the speaker issue. For me, my priorities were:
1. Key Feel (and effect, souch as after touch),
2. Expandability (memory, sounds, patterns.. you name it - I want to be able to expand on it),
3. Features (sampling, arpeggio...)
4. Sounds
5. Product Support
I got lucky, in that although product support was last on my list the Yamaha Motif6 which matched my other criteria to a T has probably the best support out there.
Honestly though... You'll be doing yourself a favor if you overlook the speakers and re-prioritize a little in what you want and expect. Let us know what you end up going with!
-Shaz