They are really similar. There are some difference in stats, but not much. The most important difference is the Shield looks. As you know, the Reman Shield has that gaudy look while the Romulan look is more subtle. Head to DS9 on Tribble and there is a vendor that sells all the Reputation items for free. You can see how it looks on any of your ships for yourself.
Romulan space set looks are barely noticable on a KDF ship (they already have a slight green to them). Unlike the Reman shields hte Romulan shields are much more subtle. The stats differences are + to att pattern skill for Romulan engine, in place of + manuvers on the reman engine? (not quite sure, I'd have to check and I'm kinda lazy right now). I tried out the romulan set on a KDf toon before LoR and was massively disappointed. Plasma torps are still slow and the capacitance HP didn't seem to do much. Admittedly its hard to figure out the exact numbers in the middle of being shot at. KHG, MACO and OMEGA are just so much better. However, if you like the Reman set well then be my guest and pay more dil for almost the same thing that you already have.
Given the buffs to stealth and detection on the Romulan shields if you've got a cloaking vessel it could be worthwhile. Or at least the Deflector may be.
I was once DKnight1000, apparently I had taken my own name so now I'm DKnight0001. If I ask you a question it is not an insult but a genuine attempt to understand why. When I insult you I won't be discreet about it, I will be precise and to the point stupid.
While this may or may not sound particularly obvious, the Romulan/Reman sets best suit (IMO) the new Warbirds. Prior to LoR they were very much a niche set - not a lot of ships used High-Yield Plasma Torpedoes before the reputation Plasma torpedoes were released. That is, the set works best on "traditional" Romulan builds: ships using cloaks and HY Plasma torps, with some Sci debuffs on the side.
My recommendation: Don't bother using it on a Federation Cruiser when you could instead be using the MACO/Fleet gear. (Standard caveats apply if you've got a specific build that works for you.)
As far as STOwiki shows, the Deflector and the Shield are the exact same, as well as the 2-piece/3-piece set bonuses. The Impulse Engines are very similar, but one stat is different:
Reman gives 20+ boost to Starship Maneuvers (more defense)
Romulan gives 20+ boost to Starship Attack Patterns (better Alpha/Beta/Delta/Omega)
Other than that, the only difference is the visuals
Was named Trek17, but still an author.
Been playing STO since Open Beta, and have never regarded anything as worse than 'meh'. I have nothing against the game/devs, nor any particular wish to talk bad about either, or praise most decisions. Still, it's better to be positive than cynical.