I dont' like these networks because they're too slow for me (actually the users are slow not the network itself)...
From Gnutella: A network of mostly small files, good for music. It is much like Gnutella2 in the way that it acts in a similar structure, but it lacks the efficiency and features. Even with more users than Gnutella2, it actually offers worse performance. It also has short queues, like Gnutella2.
Gnutella is best for smaller files (~ 4-10 MB).
From eDonkey2000: eDonkey is a network of long queues and large files. Donít be surprised if you try to download something on eDonkey and youíre waiting in queues the size of your state's population. But, the network has pretty much everything, if you wait long enough (say, leave your download going overnight or longer). You'll be able to get most files eventually. Keyword: Eventually. The download is going to be slow, much slower than most other networks. This is why the network is suited for broadband users, who can leave their connections active all the time and wait through the queues and the slow download. However, there is a credit system involved - the more you upload to a user, the faster you go through their queues, helping you download faster. The longer you leave it on, the faster your download will be - to a point, of course.
eDonkey is best for larger files (20 MB +).