Like most riders, I started Mountain Biking with NON-UST, tube filled rims. I knew the day would eventually come when mtb riders would receive tubeless technology, since it was already being used on car and motorcycle rims.
I must admit it took me many years to make the switch, but it was one of the best upgrades I ever made. Since I was riding both Freeride (FR), racing downhill (DH), Super D (AM) and putting in plenty of cross country (XC) miles on my Cannondale Gemini. I had the tough decision between an XC UST rim, and a heavier duty FR / DH UST rim.
Of course there are Pros and Cons to making this decision. If your more than 160-lbs and you ride hard, jump a lot, you might consider a heavier UST rim. If you can true the wheel yourself, then of course you could go lighter if weight is an issue.
If your just running cross country (XC), go with the XC UST.
It is hard to explain the increased feel you get from UST and the additional confidence as your control increases. Also you can run higher pressure, reducing flatting and rim damage, with the same performance as non-UST wheels which require less air psi for the same performance. Of course lowering air psi on Non-UST wheels is an invitation for rim damage and flats. Plus the tire folds over more reducing control. Often adding too much pressure to Non-UST wheels, makes the bike too bouncy.
In the end, once you go UST, you will never go back:)
For more info on setting proper UST wheel pressure (PSI): click here