hi wibem
It's always interesting how we see things differently... I had PSR-S670, which was in the same "category" as now SX600 is, and so I can talk from experience. Yes, SX600 has better voices, etc., but physically it has similar characteristics.
Hard plastic buttons -yes, they're nice to touch, however I didn't really like loud "clack" sound they make on pressing. Because of that, I just prefer silent rubber buttons.
Weight difference -ok, if carrying very often, then difference of 3kg might be important for some. But in general, SX700/900 (11.5kg) is still considered as lightweight keyboard.
Startup time -just measured and it's 17sec for SX700. Not even worth to mention.
Touch screen -how to put it... Ok, I'm glad you're happy with SX600. But everything you said about SX700/900 (big) touchscreen is nonsense.
As I said many times: SX600 is probably the best choice for entering into arranger keyboards -it has all arranger features at reasonable price. I would especially recommend SX600 to those, who are not sure if arranger is "the right thing" for them. And it's greatest value is (in my eyes), that it can also be used for "serious" performing -it's not a toy!
But being an entry level arranger keyboard, it has shortcomings. It really depends how important they are for someone -but to be objective, they shouldn't be denied.
Bogdan