The 939 will be the defacto AMD socket for the time being and it will conytinue to produce ever faster processors in that format.
754 has by all accounts ended at the 3700+.
So if you want to upgrade later on you would be better off picking the 939.
The downside is that the 754 are cheaper.
I think you will struggle to find a 3800+ with 1GB for £1100.
Mesh who normally offer the best VFM have their 3800+ with 512MB for £1400.