There's no clear cut winner in terms of price. Compare similar packages and include the line rental from Sky and I've found the difference is maybe a pound or two either way. The free HD virgin advertise is just smoke and mirrors as the total bill almost always is the same. Virgin do throttle the service back 50% during peak times which is pretty much all afternoon and most of the evening, but if your starting point is 100mbps then you're still getting a good service. If you're outside their fibre network though the performance benefits just aren't there. As for picture/sound, I'd be very surprised if there's any quantifiable difference. I've had both over the years, neither was clearly superior, and as I'm one of those that noticed the drop in quality from the BBC HD feeds I wouldn't consider myself uncritical.
If you've had bad experiences with either company then the grass is always greener. Much of the satisfaction you have after a move can come simply from not dealing with the original company anymore.Bad experiences are always shouted about louder than good ones. I've never had a bad customer service experience with sky with UK call centres and polite staff. Sky have received many awards for customer service, and they have invested heavily in HD and PVR technology, the benefits of which are now enjoyed by everyone regardless of their service provider. Despite the Murdoch connection I don't see them as the bad guys. Any time they threaten to charge for something you feel you should get for free there's always the all-powerful customer retentions department. Threaten to move and they'll move mountains to get you to stay.