Well if you're getting it for the radio then the S639F is fine, I would've gotten it instead of the S839 but the £99 price tag gets me! It's really cheap!
But I never listen to the radio so it's an easy decision for me. The real choice you have to make is:
S639 PROS: 40 hour battery life, radio. CONS: 2 inch screen, £10 more expensive
S839 PROS: 2.4inch screen, bluetooth (sleeker design) CONS: 36 hour battery life
It's a tough choice! But I don't think you can go wrong, they both seem very cool players. But with either player, an added extra (for me at least) is getting 3 months of Napster for free. I'm currently paying £10 a month for that, so if I pause my subscription for three months it's £30 in my pocket! Go Sony!
EDIT: Also, the Bluetooth headset included with the S839 is the Sony DR BT21G, which sells for £50 alone. Including that and the Napster subscription, this player pays for itself!!