While I must confess to be amongst the heathens who actually haven't used either yet (though I've had a quick peek), I have strong feelings about data ownership, and that is perhaps one of the main reasons I never used del.icio.us the first time I looked:
The access to my data - not just via basic interfaces, but in forms I can easily download in batch and manipulate or move to a competitor is a key thing for me.
It's one of the things that make me keep my main mail account locally instead of with a free mail service.
It's one of the things that makes me avoid applications that use closed file formats like the plague.
It's one of the things that makes me willing to pay for a service I could otherwise get for free
Data ownership is going to become a competitive issue as more people start running into the consequences of not having control of data that they depend on.
It's not just about a company going out of business, but conflicts (do you disagree about payments? price levels?), ability to move to a product with better features (if you're so sure you have the best offer for me, why do you try to make migration hard?) and the ability to reuse MY data in ways not supported by the platforms hosting it.
I'd happily pay extra for a solution where I have full access to data. And while I believe "normal" users probably doesn't care all that much, I believe there is a significant market opportunity for companies to provide this kind of service, as it's a potential for significantly higher margins.
And the funny thing is, while I want to know I have the ability to move easily, and while I would use the chance to back my data up on a system I have control over, I am an extremely loyal customer. I'm not price sensitive, and I HATE the hassle of moving services, and the ONLY thing I expect from a service provider that provides a decent quality service in order for me to stay is transparency - when something goes wrong I want them to tell me, and be honest about it.
That's the main reason I've stuck with my current broadband provider, for instance (if you're in the UK, check out plus.net) - their customer service has always been outstanding, and their support system is the best I've seen. They seem to be quite good value but thanks to their customer service, I haven't even been bothered to look at other prices for the last 2-3 years.
It boils down to trust. I trust Plus.net. I'll trust an online service that give me full access to the data they hold for me in a format I can actually use.
They can feel free to charge me for that - it's worth it.