Doom9 has a few analyses that use mathematic analysis of the results, but the fact is that the entire process is subjective. For example, transcoding will often result in outputs that are, in some ways, more mathematically similar to the original since they don't even touch a certain percentage of the bitstream, but a significant majority of people prefer the output of a true encoder for anything over about 1:1.2 compression.
If you really want to do a robust comparison to determine your own tastes, try selecting a cross section of movies that really strain the transcoding engine- a brightly animated film (anything Disney); a fast moving action film; a moody, darkly lit atmospheric film (but not cheap horror- you want something done by a veteran cinematographer who doesn't shadow stuff out of existence); and a film with noticeable grain (Saving Private Ryan is a good modern example). You may find that one program is better across the board (to your eyes), or you may prefer different programs for different content.
And don't take for granted the idea that DVD-Rebuilder is prohibitively slow. My P4-2.8 w/ 1.5GB RAM can easily handle a 5 (4 actual) pass CCE encode in 5-6 hours. I'm sure an Athlon x64 or a Core 2 Duo 2GB would cut that in half. If you stay at the default of 3 passes (2 actual passes), you would get pretty good speeds even without a newer CPU.