I'd go with the smaller size and run a longer stem, to match the total reach (effective TT + stem) of your road bike. Stems are cheap, and the shop might even swap you sizes for free. A longer stem handles more predictably, in my opinion. If you're between two sizes that both fit reasonably well, always go with the smaller.

Also, are you buying the frameset or the complete bike? In the bike world, the complete bike is almost always a better deal, but I'd make Surly the exception to that rule, if your eBay and mechanical skills are up to par. The stock bars (Salsa Bell Lap) are weird, the brakes are some of the crappiest on the market (Tektro Oryx), the crank (Sugino?) is an outdated square-taper model (plus it's heavy), and the rims (Alex) are boat anchors. The derailleurs are crappy too. You're better off building up a bike to your own spec.

FWIW, I ride a 54 cm Specialized Allez on the road, and my 52 cm Cross Check fits perfectly.