I thought the 90nm Winchester core was only for socket 939? I remember that S754 only had old Clawhammer with 1mb cache core and Newcastle with 512mb cache core (but higher clock speed to compensate).
To answer your question:
1) when you usually upgrade do you buy a new motherboard + cpu or just the cpu?
If you upgrade cpus 2 or more times during the span of one motherboard => S939, otherwise S754 is fine
2) If you play games, then when you pair up this rig with a card, pretty much expect that next time you replace you have to buy a whole new motherboard, cpu and videocard (in pcie). If you go with socket 939, you can get 6600GT PCIe right now and you have a larger upgrade path for videocard and cpu in the future without having to throw a motherboard.
3) if options 1 and 2 end up being too expensive today, as such their costs might exceed the cost of a socket 939 motherboard for you over an equivalent setup you wanna get. Now, imagine you were going to upgrade in 1 year on S754. You get a new cpu, new 939 mobo + new pcie videocard - Option 1. Or in 1 year you get a new cpu, and new pci videocard on socket 939 - Option 2. If Option 2 - Option 1 > cost of 939 motherboard, then you will have gained little by going with socket 939. Also if you have an AGP videocard you want to use, getting socket 939 right now might be a bad choice as well.
4) Socket 939 should overclock better.
I would primarily look at points 1-3 and make your decision based on that, because overclocking is never guaranteed.