Shimano had about 10 (?) different levels of quality. Dx is one up from lx. Not that much difference but dx is only one below xt which I think was top when this bike was made (xtr came in around that time to knock it off the top spot but is very much rarer).
Personally I would tend to go for xt or dx from that era as they are still relatively cheap and easy to find, and indicate a higher quality bike. It varies between manufacturors however and lx is perfectly decent. I've never even considered anything with xtr.
Massive thanks for the info, so the actual frames are all the same then between the DX and LX? What would the max weight of a rider be for these? I'm 21stone and would like to get a Cro in the future when I've lost quite a bit.