fenderphil, I'm afraid you got the string tension subject back to front. Shorter scales have LESS string tension than longer scales. It is easier to hold down and bend the strings on a 24 inch scale guitar than on a 25 or 25.5 inch tension. Check out
http://www.google.co.uk/#hl=en&q=24+inch+25.5+inch+string+tension&meta=&
fp=5314e9eaa343dff4
Apart from that, the number of frets on a guitar has no relevance to the scale length. A Strat with 22 frets is the same scale as a Strat with 21 frets. They are both the same 25.5 inch scale.
Right, the original poster queried 22 frets v 21 frets (not scale length!) The only difference is the fact that on a 22 fret neck, your highest note is a semitone higher than on a 21 fret neck.