Personally I think the choice really depends on how long you typically keep a computer around and on your budget.
Advantages of Socket 939:
1. CPU upgrade path beyond the 3700+ (which is the fastest current Socket 754 CPU).
2. Dual channel memory controller gives significant (5 - 10%) boost in media encoding apps. It will also gain a bit more in performance moving to a a 64 bit OS than the single channel S754 CPUs will.
3. Athlon 64 FX CPUs are not available for Socket 754. If you want the fastest desktop computer money can buy, you don't have any other choice but Socket 939.
Advantages of Socket 754:
1. There are much cheaper CPUs available. The 3000+ and 3200+ models are priced extremely well compared to the competition given their performance. Even the Athlon 64 3200+ tends to beat the best (non Emergency Edition) Pentium 4 when it comes to gaming.
2. Socket 754 only requires 1 memory module for best performance. That means if you plan on building a system with 512 MB of memory you really only need to buy 1 512 MB stick. With Socket 939 you need to have 2 memory modules to take advantage of the dual channel memory controller. This means that for a system with 512 MB of memory or less, Socket 754 will cost you less for memory.
3. Performance between the 3400+ (S754) and 3500+ (S939) is almost identical, but the 3400+ is much cheaper. This might be important for those on a tight budget.
4. Socket 754 motherboards may be slightly more mature overall and tend to be lower in price than the still very new Socket 939 boards.
Basically if you are trying to build a system on a tight budget, or if you don't plan on upgrading a CPU without buying a new motherboard to go along with it, socket 754 is not a bad choice. The fact that PCI Express is not yet available on either Socket 754 or Socket 939 also might make it more likely that you would simply buy a new motherboard the next time you get ready to do a CPU upgrade anyway.
