Accordianist - The advantage of the HD is not really the additional Styles. You can get many hundreds of MS styles easily from other user groups. In addition, although not publicised, the XD plays (most) SD1 styles without conversion - out of the 307 original ones around 20 wont play. These are all great styles although a few are replicated on the XD.
The advantage of the HD is SPEED - if you want to load lots of different dets of user styles or reqistrations or OTS or SMFs (Standard Midi Files) then this is much much quicker than from floppy.
I make do without a hard disk. I can load around 35 additional styles before running out of memory and that is enough for me to gig out with. When I'm playing at home then it doesn't matter if it takes a little longer to load stuff. I may get extra memory in future which is a cheaper option but at the moment I don't need it.
Hope this is useful for you - whatever you do though get an XD or SD1 if you can afford it.
KF
Hope this helps