I haven't joined a gym or health club either. But the difference I see is that a gym/health club has wide open hours and many services (spa, pool, etc) as well as classes so I could find something to utilize even if were injured or was working odd/extra hours, etc. But I'm not interested in a gym at this point. I just want to try aikido, not sign my life or my bank account away to it.
Totally understandable. You need to know that the dojo is a good fit for you and you a good fit for the dojo and that is hard to commit to getting to know them if you are going to be locked into a contract. I offer students a couple of free classes before they make a finanical commitment, though two free classes is hardly enough to figure out if you and the dojo are a good fit. I don't require contracts but I understand them. Good luck with your search.