With Blurb you download software, do all the work on your home PC and then upload the book for printing, so it is not held on the site of the company, nor is it prone to server issues(until you upload it).
Altavista print makes you do all your work online, so if there is an issue you can loose all of your work. (I find the ones that make you work online quite slow and frustrating). This is what makes the sites that ask you to download their design software much superior.