If you are prepared to strike regularly then join the NUT but remeber you will lose a days pay which is calculated as a percentage of all the working days in that month. (Expect it to be around £100 if you're in your first 3/4 years of teaching.
Join NASWUT if you want a strong union that isn't too militant. I've often found them to be quite firm and level headed on the big issues.
Join ATL if you are never prepared to strike. I think they've only ever taken strike action once. A good supportive union but often the smallest number of members in a school. If your egaer to be a union rep at your place of work there's a good chance you could take on the job with this union.