First: Put your paper on the arxiv. It's a preprint server. Then the work is "out there" and you are still free to submit it to any journal you like. So my short answer is: do both.

First: Put your paper on the arxiv. It's a preprint server. Then the work is "out there" and you are still free to submit it to any reasonable journal (as noted in the comments, not every journal takes papers that are already on a preprint server, so check that in advance). So my short answer is: do both.