You are right, Vodafone is more expensive. The problem I had was finding a phone with the specs I wanted on Virgin. I almost ordered the K700i, but lack of memory expansion put me off. Also, once I started looking into that phone, it seemed that a lot of people have had problems with it. Even when I went to Carphone Warehouse and wanted to view one they said they wouldn't be getting any more because of the problems with it. But I bet lots of people are also happy with it....mind you, it is also a 2 year old phone.
So basically the only phone I came accross with the right specs and price was the E770, so I switched to Vodafone. Worth the extra ? don't know, maybe depends how much you use it, but as I mainly use it for incomming calls, i'm not that bothered.
Vodafone Live is ok for some things, like I find the football info good (but is subscription based for videos) but a lot of the downloads like wallpapers and ringtones are a complete rip off ! just download them on your pc and copy them over for free
If you can find a phone with the specs you want, then probably best to stick with Virgin......