I think they are both useful in that it is better to have access to go problems than not to.
Some advantages of online / computer:
1. If diagrams are too small, it is easy to magnify them.
2. Although the online sites don't always do this, a computer program such as uliGo can randomly rotate problems or switch colors. I don't think this matters much for anything but doing the simplest problems for speed, but some people like that.
3. Some online go problem collections are free.
4. Easier to represent multiple variations / refutations. Books usually just give the main line and maybe 1 variation.
Disadvantages of online / computer:
1. Easy to get into a mad-clicking habit that isn't productive.
2. Need to have internet access or computer access.
3. Some of the online sites have mistakes in their solutions.
Advantages of books:
1. Some problem collections are only in books.
2. No requirement for power, electronic devices, or internet connections.
Disadvantages of books:
1. They cost money.
2. As physical artficacts you have to have a place to keep them.
3. While 1 book is portable, a collection of 100 books is not.
4. They can get worn out. (Although, for some this might be a point of pride.
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