Hi Akshat:
Any of the 3 brands you mentioned are great. You can shop within your budget with confidence and get a good MIDI workstation.
Roland is one of the oldest names in MIDI technology, wave-table/synthesizer chips, and drum machines. A friend has a Roland G-1000 workstation (used Zip disks and SCSI drives for storage) that still works well (the more-recent models would be the V-Arranger series: VA-5 / VA-7/ VA-76, or the E-series, like the E-09 @$600USD, or the GW-8 @$1000USD). I still use a pocket PMA-5 Roland gadget (it was a "MIDI PDA") for quick arrangements. "Boss" and "Edirol" are sister-brands you might have also seen.
Korg is a well-respected synthesizer / keyboard maker, and many top touring bands specify Korg instruments for their keyboard players to use in concert. The Korg PA-50 / PA-500 are in the $900-$1300USD range, and the Korg microArranger is around $600USD. These are all 61-key units; when you get into 76- or 88-key units you get into the $2200-$4000USD price range.
Yamaha you know is good at keyboards (their classical grand pianos are among the best). Any of the Yamaha PSR-series 61-key arrangers are in your price range: PSR-S650 @$800USD, PSR-S710 @$1100USD, PSR-S910 @$1800USD.
If your interests lie in Middle Eastern, Arabic, and Mediterranean musical styles, I'd recommend Yamaha's PSR-OR700 Arabic model which has Arabic, Iranian, Turkish, Greek, and other World Music styles in addition to regular Western/Latin/Rock/Jazz styles. It is in the $1300USD price range.
hope this helps,
--Dennis C.