Well, a lot of varied opinions apparently. The reason I like the 39 bracket has already been mentioned, everyone above lvl 35 gives you a run for your money. It's fairly balanced and you can seldom guess the outcome from the start. Ally side has picked up the pace and we are about 15 strong at the moment (active twinks) and another 10 that logs on every now and then. On the horde side I'd reckon its 20+, with about 12 active. I never really likes the 29 bracket, had a little fun, but as a mage you really don't "bloom" until 30+.
The competitiveness is nice in the 39. No bad blood or rude gestures while playing

if you are planning on twinking at 39 on alliance side, there are 2 guild to chose between, Godlike and STVs finest. The latter one is new and a bit more active. Old school twinks with green lens of eagle are in godlike. Hope to see you on the battlefield

Heliosone - 60 priest
Mypreac - 39 mage