I do own a mid-80s De Rosa Professional. I've never ridden a Somec, but I've gushed over a few of them. One of the local shops had a display model with custom leather wrappings around the tubes--I imagine for urban protection--with custom stitched "Somec" lettering/graphics on the main tubes. Black paint & leather wraps with chrome lugs. It was a really attractive bike.

I'd expect the ride and craftsmanship to be very similar, but Somec may have some flashier details (extra chrome, internal cables, BB cutouts, engravings, unusual color schemes and graphics, etc.). All else being equal, I might give the slight edge to Somec for their tendency to include these extra embellishments and because I prefer brands that are a little more obscure. However, the pantographed Record parts package on the De Rosa might put it over the top. Classic first world problem.