definitely test for different sizes in each model your interested in...!!

I tested the 17" and 19" Giants Boulder SE and Rincon, both were great.
I could tell the 19" would be best for road riding
and the 17" would be better for trails...
there was still a similar feel between the 17" and 19"

I also tested the Trek 3700 18" and it's geometry was perfect for anything I would want to do...

Now the 18" Trek is supposidly between the Giant 17" & 19" models, but had a huge difference in feel....

(I ended up getting a 17" Giant Rincon, wanted to persue more trail action)
http://www.bikeforums.net/showthread.php?t=107768

Back when I bought my Moab in 95', the bike shop where I was purchasing it from let me upgrade to the newest gripshift and tires. The ones that they removed were decient quality and they credited the cost towards the upgrades, since they new that they would use them to upgrade someone elses base model with junk....
just a thought...

Find something strong that feels best, and upgrade what components you feel need, or others suggest... ie. the rims. Some MTB have an aggressive tread which different tires can be put on too...

Hope this helps other than improving my terrible typing.