I would personally go for a 300 (or higher spec) Roland. I personally feel they are better made and tend to be built better. That is just a personal opinion though. I will back that up with the fact that I know loads of people that have switched both ways. Those that go Roland to Mimaki tend to regret it, those that go Mimaki to Roland don't. Again, just something I have found as I do not own a Mimaki (anymore) this is a very limited view.
Also, note that if you intend to print bumper stickers you should laminate them. Stickers for outside use really should have a UV protection.
If you want to do stickers I would consider an OKI and GRAPHTEC cutter automated process. I think they are much better for that sort of thing.