Monster truck and 1/8 are the two biggest nitro classes around here. MT looks like a ton of fun but I can't handle 3 different nitros. 1/8 is awesome if you can control it and know the basics about getting around the track safely. 1/10 gas truck is I think the most fun to race if you have a full class and can throttle control the 2wd. I think gas truck can teach you how to be a better racer as you have to learn control otherwise you end up hating your truck. If you can get a gas truck around the track decent then you should be able to handle a mt or 1/8 with no problem. But they are all different to drive and I'd have to say MT must be the easiest because they are pretty much self marshalling.
Gas trucks are the easiest by far to maintain and work on by far. Much, much less moving parts, and the diffs aren't oil filled just mechanical. Except the new jato.