Cyclocross - how should a CX bike fit me (compared to a road bike)?

I'm thinking of getting a Motobecane Fantomcross Uno & trying to read the sizing chart (which, if you are familiar w/ the site is a bit of a math exercise because you add an inch to the standover & the convert to cm, etc.) and compare with my 'classic fit' steel framed road bikes. Both my bikes are about 55cm w/ a top tube length of about 53/54cm and about 81cm stand over height. (I realize I'm leaving off some measurements & angles here) One actually has a zero degree stem, so the drop bars sit at a position you may consider more common to a cross bike in the first place. Both fit well, as far as I can tell & by traditional methods of measure. My question is should I be looking for a frame that fits with the same stand over height, or should I want a bit less? My take on the sizing chart at BD is that a 52 frame is going to be about the right fit...standover with a bit more room, top tube on par with my road bikes, but a slightly shorter seat tube & thus (I assume) more leaning over on the bars. My road bikes fit with what I call more of a 'classic' fit...ie, just a little more than a fist's worth of seat tube showing. I guess my question really revolves around whether I am better off with a fit that calls for a bit more seat tube extension (and maybe/maybe not a little lift in the stem) and some more room at the top tube, or if I am better off with a fit like my road bikes.

I would gather I should plan on more seat tube extending, but I thought I'd ask how folks like their CX bikes to fit.

For me with this particular bike, this translates to going from about a 55 in my road bikes to a 54 or 52 for the Motobecane Uno. I don't have a feel for whether this sounds about right or if I'm looking at getting the "DUI bike" fit. I'm about 5'9" btw, as a point of reference.

My CX bike has the same dimensions as my road bike, with the exception that the head tube is 72.5* instead of 73*. The stem is 10mm shorter and has 10mm more of spacer underneath, to balance my weight comfortably for more technical riding. It's still a fairly aggressive looking position (bars 7cm below the saddle), because I have monkey arms.

Though I typically run a bit more seatpost than you do, I haven't found standover to be an issue with the CX bike, despite the fatter tires. This includes many miles on singletrack and rougher trails where you would think standover would be a big deal. In practice, it's just not. Most unplanned get-offs are to one side or the other, or over the bars. Rarely if ever have I come straight down on the top tube.

For your specific situation, I would recommend getting the size that allows you to get the bars to the correct height without a ridiculous spacer stack. I might go with the 54 for that reason.

A 54cm road bike typically fits me well, and I dropped down to a 52cm for my Fantom Cross Uno and it fits well. I'm 5'9" as well.

I think it depends on the geometry. On a road bike with a non sloping top tube, I ride a 54cm. My cross bike has a sloping top tube and I ride a 56cm. 110cm stem on both bikes.