The 990XL is an actual large diaphragm condenser. The 990 has a smaller diaphragm, and I guess would be called a medium diaphragm? I think that I got my 990 for about $60 on sale at MF.com 10 years ago. Man they must've bought a crap load of those mics!
The 990XL usually goes for $200. I've never heard one before, I'd be curious how it stacks up. The 990 isn't all that great, but compared to other $60 condensers it's not bad at all.