Spark debate or war? I'd side with Virgin Media catchup because of the methods of streaming but in a way it's like comparing apples and oranges, on the face of it two similar systems, but very different ways of working.
Indeed- I've very limited experience with the Virgin Media platform- the last time I did use it (at least a couple of years ago) I thought the selection was excellent, but the interface (much like the rest of the box) was sluggish compared to Sky+. The initial 'Sky Anytime' was a reasonable selection (with lots in HD) but very limited. Since 'Anytime+' has come along, its much more even- especially now that Anytime+ is available to any Sky customer with broadband, whether or not its Sky's broadband service they have.
In terms of 'catch up' the Virgin Media definitely have the edge at the moment with BBC iPlayer and 4OD, however with iPlayer and ITV Player coming soon to Sky that will be a much closer call. In terms of archive Sky are getting there slowly- the selection of archive programmes is getting better bit by bit. They could do with more Warner and Universal TV content that Virgin has, but they've got some good stuff from BBC Worldwide, Fox and others. The big omission from Sky is Channel 4 archive content- which I seem to remember Virgin having a huge selection of. Where Sky do have the advantage is if you're a movie subscriber- the selection of films is huge, the only problem is not enough of them are in HD.
The fact that Anytime+ is delivered over ADSL isn't an issue for me- an SD programme is available to play almost instantly and an HD one very quickly.
Both services are very good- Virgin probably still has the edge because its been running a lot longer, but Sky are catching up fast even if theres still a few omissions.