the problem in comparing geometries (at least in terms of tube lengths) is that the paddy wagon has a sloping top tube where the rob roy has an almost horizontal (i'd say about a half centimeter drop) top tube. since the paddy wagon's tt is sloping, and it comes in at 56.1cm, i'm guessing it's a little bit longer of a bike than the rob roy.
that being said, some manufacturers release an "effective" top tube length which roughly translates to the length the top tube would be if it were horizontal.
in terms of which would be more similar (and anybody please correct me if i'm wrong here) i think the 58 (which has a 54 c-t-c seat tube and a 56 top tube) rob roy would match more closely than the 56 (which has a 52 c-t-c seat tube and a 55 top tube).
unfortunately, this is all largely academic. i've never calculated my fit nor been fitted by a shop, i've just had the good fortune to ride lots of different kinds of bikes and figure out what i like based on that.
on the off chance that you find yourself in the dc area you're welcome to take mine for a spin. best of luck.