I'm in the "ride what you've got for a while" camp, unless you've got a big budget or come across a great deal. You'll find much better deals in the offseason, waiting gives you time to try some different sleds, and you can learn a lot on what you've got. I just rode my "ancient" 600 last weekend. I'd learned some things riding my "new" sled (a '12 Pro RMK), but was surprised at how well that transferred back to the old sled. I could do things that it took me a season of riding the Pro to get comfortable with. I've never rode an M8 like yours, but if people are saying it's not hard to sidehill, then it's not like you're going to wear yourself out and hit a wall. Sleds like my old 600 are a PITA to get on edge in stock form (I've narrowed it and made some other changes that help), and the newer sleds are night and day better, but you're coming from something much closer to my Pro than that.
As for what next, I'd avoid long tracks (17x) and turbo or engine modded sleds. Probably a 163 is the sweet spot for you, but a 155 might be better if you're not in deep stuff much. The trouble with a 17X sled is it's going to want to go straight and be more difficult and frustrating to maneuver; you only want the extra length if you need it, and you don't around here except for a few epic days a year at Cooke. At this stage, a turbo would probably just "help" you get in more trouble, and an aftermarket one could be a ton of headaches. You'll know if that extra power is something you can use after you've rode more, but right now it's not likely you'd even use the extra power. One plug for the AXYS: it's not the easiest to sidehill and carve with, but you can do about anything to it to and make it more playful, more or less "tippy," and so on. If you like working on your own stuff and tweaking things, Polaris is probably where you want to be. Doos tend to be more pull and go, but heavier and harder to work on. Cats have their issues, and the monorail is kind of a love it or hate it thing, from what I gather; others can speak better to that. You can spend days reading up on one particular sled here, so especially if you buy used, do some digging and you'll probably get a good idea as to whether it's a problem child, or not the right thing for you.
One other thing on learning to ride: push yourself, but don't force yourself. Because of the particular conditions, something that came easily one day might seem impossible a week later. Nothing will ruin a day like forcing yourself to work on something that doesn't want to come. The better the snow, the easier it is to hold an edge and make the sled do what you want, up to a point. Also, it's still hard for me to look at terrain and judge what I can do. So, on crusty days, or if you can't seem to hit a sidehill, just move on to doing something more fun and easy, and maybe try it again in a different area. Obviously, you have to challenge yourself to advance, but it's easy to overwhelm and/or wear yourself out. When it clicks, you'll get far more comfortable and make a lot more progress in an hour or two than days of trying to force it.