I'd say trails will be fine. Mowing the fields - depends how often, how much field, how rough they are, and how long you're prepared to spend doing it. The B2650 will do it. An MX would do it a lot faster with a bigger brush hog. But if you're mowing the same fields often then you don't really need a brush hog, and you can run a wider flail than you can a brush hog. As always, these things are relative - hiring it out would also save you a lot of time, and might be cheaper than upgrading the tractor. Most of us would say "but I like driving my tractor" - which is also in a sense the answer to whether you need to upgrade - if it takes a bit longer but you like driving your tractor, then there's no problem.