I like both books, the Brit Guide, apart from being from a UK perspective, is slightly more for newcomers, whereas the UG goes into far greater depth for the seasoned WDW connoisseur. I also think the UG is a bit more critical.
If I were you I'd get both. I tend to buy one of each alternate years.
Well, talking of Play, I found WHSmiths online to be the cheapest for the Unofficial Guide. However, I choose the Store Delivery option to get free postage and it was a farce so I wouldn't recommend it! Can't comment on their Home Delivery though.
I'm also currently finding they're the cheapest for pre-ordering the Brits Guide as well.