Well, actually there are complaints about both of the droids from the hardcore prop enthusiasts. The R2 builders think that the Sideshow R2 is a piece of junk. But that's because they're coming at it from a very different perspective. Their hobby is making replicas of the R2 units as they appeared on screen, and this generally means trying to replicate the original parts and so forth. Sideshow's R2 isn't a replica in that sense -- it's a life-sized statue. To the average person (and even to the average hardcore Star Wars fan) they look great, but if you want to start comparing each individual part with the ones used on screen, you're going to find differences. To me, as long as they look good I'm not going to stress about whether a particular part is 100% screen accurate.
To me, the only difference that is noticeable is C-3PO's eyes. They appear to be a bit too big. However, it seems that the way the figure is lit can greatly affect how it looks in photos, so it may be that the size of the eyes is being exaggerated in some shots. We don't have that many pictures of the final C-3PO, but he looks quite a bit better in the shots from SDCC than in the official ones on the Sideshow web site.
In any case, the only options for life-sized droids are to build them yourself (NOT an easy task, especially when it comes to C-3PO), to track down one of the Don Post versions from the 1990s (probably a difficult and expensive proposition) or to get the Sideshow ones. All things considered, I think they're the best option for most people.