I think the mic input is teh best addon in the 2100, but the usb?SM card is teh best part of the 1500.
Sonically - they are similar, but the drums are better in the 1500, and I guess all the new styles will work in either.
There are definately some better sounds in the new stuff, but the difference is not drastic.

If you sing, or want to play aguitar along with this instrument - the 2100 will be a better choice for a home user.