So I really don't know which one to do. I like to learn riffs from songs because (like all guitarist) it gives me a buzz when I play a song that I have listened to. But some people think that this is a waste of time. Randy Rhodes once said,"Why should a learn anybody else's riffs, what am I gonna do with them,". I think that learning some riffs are a really good way to learn form. If you keep learning the same difficulty of songs and never challenge yourself then you will never become a better guitarist. If you do challenge yourself though, you can become alot better than just by practicing a few finger excersizes. Yes, they are helpful, but they are just really boring. A riff gives you all of the excersize you need (as long as its a good difficulty) and plus you learned to a new song to show off to people. Some people like a good mix of both, but I guess it's all just personal preference.