Personally, I’d pick DeRita, mainly because the act felt more like the Stooges when he was around, even if it was more of an older version. Besser was funny on his own, but the problem is how much his personality doesn’t fit in with the act. It’s like if you were to replace Oliver Hardy with Moe Howard. Both have shared the same material (notably THE LAUREL-HARDY MURDER CASE and IF A BODY MEETS A BODY) and have some similar characteristics, but Moe would come off as too cruel to the child-like Stan, as opposed to the more idiotic Stooges. Besser is a similar case. At Columbia, he used some of the same material as the Stooges, but his bratty man-child comes across as very annoying and it often felt more like Besser was doing his own separate thing from Moe and Larry.
I’m sure DeRita would have been better had he joined at a better time. He actually was pretty good on the Stooges’ Steve Allen appearances. Still, I choose him anyway because he seemed to fit better and the act felt more like the Three Stooges as a result.