You might want to go to the Photography Forum -- you'll probably get some great answers --- might even get more info than you thought you wanted!
Photobucket and Shutterfly serve different purposes.
Photobucket would probably be considered more of a photo storage and sharing site. Once you store a photo there, you can link directly to it in an email, in Forum messages, on a website, etc. I'm not absolutely certain of this, but I believe that you can add Tags to photos. You can use a designated amount of space free. If you decide that you need more space, then you can pay for additional storage.
Even though you can store photos on Shutterfly, that is not its primary purpose -- it's more of a photo processing site. It is primarily for you to purchase photo products ---- prints, photo books, mugs, etc. If you store your photos there, but you do not order anything within a period of time (like 6 months), you MIGHT get an email after that period of time warning you that your photos will be removed. You can share albums with friends via email. I don't believe that you can link to individual photos --- sharing would be of an entire album. Also ---- I am not absolutely certain of this, but I believe that photos stored on sites like Shutterfly are NOT stored in the same high resolution as the original from your camera. Because of this loss of resolution, I would not choose to use a site like Shutterfly as my primary photo storage location. Similar sites are Snapfish, Wolf Ritz,
Walmart, Walgreen, etc.
Don't get me wrong --- I love sites like these for ordering photo books and other photo products. But I would not rely on them for THE place where I keep my photos.
I am a person who is very careful about using the words "never" and "always". It is my opinion that if your photos are important to you, then you should ALWAYS keep copies of your original high-res photos in more than one location. Even if you put them on one of these photo websites, I highly recommend that you keep copies of your original JPG files somewhere else --- on a hard drive, on CDs, somewhere.