Having spent 5 years working for The Carphone Warehouse (not now though
) I'd say the Sony Ericsson.
Reasons:
1. You're used to the menu system
2. Better quality camera
3. Much, much, much better software running the handset
I used to repair phones as well as sell them and the amount of Nokias in for software faults was astounding, especially within the first 6 - 9 months of launch. Nokia are reknown for using the consumer as beta testers for their software, in other words you find the faults, tell them about it, they then rewrite the software to fix it (which invariably causes other faults to crop up)
At the end of the day though it's all down to personal choice and what you want function-wise, no handset will do everything you want and none of the things you don't.
Best advice would be go and have a play with them both, any retailer worth their salt will happily let you have a fiddle first.