If you don't already know enough to tell the difference yourself, then perhaps it doesn't matter at all. Just pick one and ride the crap out of it and maybe you'll know and understand what any differences might mean to you for your next bike that you pick.

I'd think the fact11 frame will be lighter, or at least I'd imagine it to be lighter. However the Roubaix Pro I looked up only had a fact 10r frame as did the Roubaix Expert. The biggest difference between the two being the Pro is 12 speed eTAP AXS and the Expert is 12 speed Ultegra Di2. However the Pro did have smaller rings of 46/33 and a 10-36 rear compared to 50/34 and 11-34 rear of the Expert. So if you are new to cycling and your legs aren't strong, those smaller rings and wider ranging rear cassette might help you if you ride a lot of steep hills.

Though I might have gotten the wrong bikes to compare if there are other models variations of Pro and Expert Roubaix's.