As refraction said you shouldn't see a lot of difference just general surfing. As for the price difference, for just stand alone broadband it's; £ 25.00 / month for L broadband (4Mb) which is soon to be 10Mb (free upgrade)
http://allyours.virginmedia.com/html...oadband_l.html
If you have other VM services you might well find it cheaper.
Virgin Medias `Where's my upgrade' weblink: (doesn't seam to be updated much though)
http://allyours.virginmedia.com/html.../schedule.html