I don't know how you clasiffied Keyboard & portable keyboard (as some of the synthesizer also quite light weight...).
Normally I classified it as MIDI Controller, Entertainment Keyboard, Arranger Keyboard/Workstation, Synthesizer.
Generally, MIDI Controller are those keyboard without any sound library, you may need some plug-in board or connect it to computer.
Entertainment Keyboard are normally with a lot of preset rythme pattern or arpegiotors,some even have keyboard with colour light. mainly for entertainment (like those some of those CASIO leyboard).
Synthesizer normmally allows you to tweak the sounds available. some of it have build-in sequencer and with Sampling capability.
Arranger Keyboard/Workstation are some how between Entertainment keyboard and synthesizer.
the choice is very subjective, depend on your needs.
normally people will choose Arranger Keyboard or synthesizer. MIDI controller may be one of the choice but you may need the sounds library.
I don't think entertainment keyboard will suit your needs in this case.
Sounds wise, Korg may tend to those electronic sounds but Rolland more thend to orchestra sounds. Yamaha are somewhere in between Korg & Rolland. (you need to try it out to see the different)
Of course, there are some other brands available.
You can go to the shop (Music Plaza @ PS, Swee Lee, City Music, ... etc) to try out those keyboard yourself to see which one really suit you.
you can looks for some shops under the dealer link.
http://www.soft.com.sg/2006/modules....viewlink&cid=4
if you having tight budget, maybe look out for those 2nd hand keyboard.
there might be some good offer. what is your budget?
or maybe you should list down your expectation on keyboard in more specific so other people can give you some suggestion.
BTW,
Normally keyboards which have in-built speakers may still have audio-outputs. but it varies from model to model.
improving Keyboardist skills?
hm... See more... read more... listen more... practice more...
Or you may go attend some classes if you want.