All of those will let you set security so that only some people can see your albums. I've never really tried snapfish, I (as said above) absolutely despise flickr, and really like picasa.

Picasa allows you to organize photos on your harddrive and select which ones you want to upload to their webalbums. When you upload photos to their album, picasa resizes them smaller and they won't be fullsize on their web albums. So it is not permanent full sized archival. I think flickr might store full size photos.

Picasa when the photos are on your harddrive, you just highlight the pics you want to print, and click order prints and it will pull up a page that will allow you to choose which photo printing service to use.

My current favorite photo printing place is adorama.com. they do have an archival function where they will store however many full sized photos indefinitely for you - I think if you pay extra you get more space. But you can also share albums from them. Adorama uses kodak endura archival quality paper - and the lustre paper is just spectacular. On thickness of the paper and quality of the print, it really just blows all the other places I've tried away. Periodically they have sales where it's $.99 cents for an 8x10 or $1.99 for an 11x14 or $5 for a 16x20 or $10 for a 20x30... usually when they have a sale, they allow you to "pre-buy" a bunch of photos, so you can print photos later at that price.