I would say read them both. With the Belgariad you're going to have to forgive a lot of things that have become cliche since that series was written. I would say that those books, if they'd been written today, would probably be YA.

The Runelords was a fun series, and I certainly was turning pages quickly, finishing the first three or four books in about two weeks, but after that I was just kind of satiated with the whole thing. I haven't gone back to that universe for anything else he's written, or to re-read what I already own, and I doubt I ever will. On the other hand, and probably because I have a childhood attachment to Eddings (my mother read us all his stuff as little kids), I re-read his stuff every few years just for the good memories.

The big knock against Eddings, of course, is that he essentially wrote the same story over and over again, and that's pretty close to the truth. In some ways, depending on what level of reading you like more, I'd be tempted to suggest the Elenium instead of the Belgariad. It's just a little more adult in nature, and I find myself feeling like the characters were a little more original than the ones in The Belgariad.

So yeah, there's my opinion for ya