Easy - The XLM. If it's $100 more than the Mountain then it's far better value for a start. The XLM frames usually go for around $200 more than the Mountain, the XTR bits are about the same price or maybe slightly more for M900 and the XLM has a SID which is $200-$300 on it's own. There's a Bontrager Ti bar on there as well.
The XLM is also said to have a springier feel than the Mountain although I can't say I noticed much difference between them. I remember an interview with Rob Vandermark I think it was who was saying that when the Mountain frame went from straight stays to the bendy ones it became stiffer and so when they designed the XLM they wanted it to feel like the original, something that was achieved by using larger diameter stays but with a thinner gauge of tubing. It's also lighter, looks better with the thicker stays (imo) and has a nicer canti hanger.
As for the age of it, I'd say 97 or 98 as in 98 they started fitting headtube badges. The SID and rapid rise rear mech were in the shops in 97 so assuming that's the original build it's easy enough to date.