The short answer? I think the Hydrovance. Now for the long answer...
Urea is a good humectant, found in the stratum corneum of our skin. It is a colourless, odourless (or just about odourless), hygroscopic ingredient that is freely soluble in water. Urea can increase skin permeability of certain ingredients - not all the studies support this, but it is plausible - and it could increase resistance of the skin to surface treatments that might be irritating. It generally comes in crystal form, so you'll have to mix it with water before adding it to your creations. There are some formulating issues regarding pH, preservatives, and thickening, so you'll want to do some playing around to see if urea will mess with your favourite lotion recipe.
Hydrovance (from National Starch or Azko Nobel) is a good humectant as well. It comes in a liquid form, which makes it an easier ingredient to use in your formulations. You can use it at ambient temperatures, which means it's a good ingredient for creations you don't want to heat up like toners or sprays (although I always recommend heating and holding!). It, too, can mess with your pH, so you'll want to get a pH meter if you're going to be using it in a lot of different products.
The strength of a humectant is dependent upon the ratio of hydroxyl groups (the OH groups) and the carbon atoms. Hydrovance has been synthesized to contain a hydroxyl group (the left end of the chain), so it should be more hygroscopic than urea. It's hard to find information on the differences between the two that aren't generated by the company, but it appears Hydrovance will be more hygroscopic than urea.
Does this mean it is more moisturizing for our skin? Possibly. (Sorry for the vague answer, but there simply isn't enough information out there for me to feel comfortable making anything more than a general comment...) I think it comes down to skin feel and preference in formulating.
If I were to choose between urea and Hydrovance for my formulating needs, I'd go with Hydrovance every time. It seems to be easier when formulating - it's a liquid, it's less likely to mess with your pH, and it's not much more expensive than urea. (I really hate having to dissolve things!)