This is a big decision and definitely not an easy one with all the promotional information out there. I think you've got the right idea by asking here--other teachers will have the most enlightening advice. You can also check out Rate My Teaching Agency (link in profile), which has reviews by teachers on many agencies. Registering with sever agencies can definitely help get you more work so long as you manage it well. Some agencies do not like this so try not to overstretch yourself by joining too many. I would recommend trying a few out and sticking with one or two of your favourites. Good Luck!