I've used both - Lovefilm does have a much bigger selection of older and foreign films, TV series, etc, although when I was with them they weren't good at sending out new releases.
I wasn't with Blockbuster for long so can't really compare in detail - I thought their packaging was too bright and had one of their discs nicked in the post, which was a bit of a hassle to report, so didn't stay with them.