939 is newer with the above advantages.
754 is a little less expensive currently, and an equivelant-rated CPU (i.e., 3200+ to 3200+) will perform a little faster on 754 than 939 (but just a little). Processors of the same clock speed (i.e, 939/3200+@2.0ghz vs 754/3000+@2.0ghz) shows 939 is faster.
754 is as fast as its going to get with the 3700+ as AMD said they won't be releasing faster CPU's to the 754 lineup. 939 CPUs are said to overclock better than 754 as well (but neither are overclocking slouches).
If upgrading piecemeal (or the ability to) is important to you, then 939 is your only choice. If you stick with a particular system and don't change it without effectively replacing it, then 754 gives you a lower price today.
Mike.