WB was the better by network by far...up until 2002. They managed to get rid of all of their good shows, and replaced them with crap-tastic sitcoms, in a desperate attempts to find another "Reba" but had no luck. Throw in a slew of ratings-challenged dramas like One Tree Hill, Everwood, Tarzan, The Mountain, Jack & Bobby and you have your answer as to why they went downhill. Although The WB has had more successful series overall in their run and had a bigger fanbase than UPN, I believe that they needed this merger more than UPN.
The WB went to hell, and it was only going to get worse. UPN, although it suffered a slow few years was on the rise. With America's Next Top Model, it had its first reality hit-- something The WB was never able to do. Not to mention Smackdown, which also does fairly well in the ratings. And with Everybody Hates Chris, they were able to do something The WB was never able to do-- find a good sitcom. Sure they had Reba, but that show is in its final year and only got a mid-season pickup. Notice no other WB sitcoms made the CW cut. The only category The WB has been more successful in is with its more popular dramas. Gilmore GIrls, Supernatural and Smallville, vs UPN's one drama, Veronica Mars. But even then, 2 of the WB dramas are most likely in their last seasons (Season 7 and Season 6) , while Veronica Mars is still in Season 3. With the exception of Supernatural, The WB has had trouble finding a new hit drama in recent years. They were running out of steam, and once Gilmore Girls and Smallville would have ended, they would have been compltely lost. (You can't run an entire network with One Tree Hill and the elderly 7th Heaven, which will probably never die). So my point is, that while The WB has had a good run, they've finally run out of steam, whereas UPN has just found its footing and if The WB and UPN remained seperate networks for another 5 years, UPN would have pounded The WB in the ratings more than they already have been for the past 2 years.