Best way to pay for Live in UK

[Reasonable]

Okay my fellow forum members, I'm thinking this December, under the guise of Christmas presents, I'll finally switch from being a 2 console owning gamer to a 3 console owning gamer and get a 360.

So, what's the best way to pay for Live?  I'm thinking pre-paid cards look cheaper overall vs direct debit, as a quick scour of the web would seem to confirm this - but, what say you longtime 360 owners?  Is this the best approach.

Also, anything I should know regarding spec, etc?

I'm going to get the 250GB Slim of course and plan to use WiFi as I live in a very old house and rely on Wi Fi for all online.