Hey, rogerwilco. Still looking?
I would personally recommend
the Alesis QS6.1, as it is a top-quality synth marked down to a reasonable price. However, the price I was thinking of was $499.
The QS6.1 was this price until a month or two ago, when all of the online retailers (and Guitar Center) upped the entire QS synthesizer line's prices by $200-$300 each.
I own its big brother,
the QS7.1, which (I believe) has the same sound set and keyboard action, and I'm very happy with it. If you can find a QS6.1 for $499, or don't mind spending the extra $200 online, I don't think you'll be disappointed. However, as a precautionary note, keep in mind Alesis's future and direction as a company is a bit uncertain right now.
Of the other boards listed, assuming you don't intend to get too deeply into voice editing, aren't picky about keyboard action, and don't care about expandability, the S03 is probably the best of the group. It has a lot of voices (700+) and is a bridge between Yamaha's consumer and pro-level boards. However, the sound quality on
the S03 varies from voice to voice. If you have an extra $200-$300,
the Yamaha S30 is a much more full-featured board that includes on-board editing and card-based expandability. It only has 256 voices, but they are all high quality.
I tried out the XP-30, but I felt it was far too expensive for what it had to offer. Though it has tons of voices (1200+) and room for two more expansion cards, I was not impressed by the sounds themselves. However, sound is subjective; try it and decide for yourself. I also wasn't very fond of the XP-30's controls, especially the little combination pitch/modulation controller on the left side of the keyboard. The final factor that tied it all together was the nearly $1000 price tag, which is very expensive for 61 unweighted keys.
I was warned off of
the X5D (old board, outdated sound set), and I haven't tried the RS-5, so I really can't give you a recommendation either way on that particular board.
A few pointers for buying a keyboard:
(1) Try all of the keyboards you are looking at. Mars Music and Guitar Center should have most, if not all, of the ones that you have listed here.
(2) Listen to the quality of the sounds on each board, especially the ones you intend to use the most. If you can, bring your own headphones, and listen to each board with them. This eliminates differences in the speakers at the store.
(3) Don't fall into the "more features = better" trap. If a board has the features you want, and you like the board's sounds, then it's probably a board you will be happy with.
(4) Don't fall into the "more voices = better" trap. Chances are, you'll be better off with 256 high-quality voices than with 1000+ mediocre-quality ones.
(5) Read up on all of the boards you are considering, especially with regards to the features you are looking for.
(6) Take your time, which you seem to be doing. Rushing into a keyboard purchase isn't a good way to go, since you will probably be using it for a while.
(7) Some "basic" specs you should look for: 64-note polyphony, at least 61 full-size keys, and General MIDI compatibility.
Best of luck.
So Is 
