I would go with Aggron for this one. For One, it boasts a great base 180 Defense which helps with Lance's Gyarados, Charizard, Aerodactyl and Drangonite's STAB Move. Also, it has a usable base 110 Attack. It has a good Defensive Matchup against Koga and Will as well. Now, it has solid STAB's in Rock Slide/Stone Edge and Iron Head. Aggron has good coverage moves Dragon Claw, Earthquake, Brick Break, and Payback. The main thing that is bad is that it Evolves at Level 42 though. Also Roserade only really gets Normal, Grass and Shadow Ball for attacking moves, which doesn't take advantage of its 125 base SpA. I think Aggron is the better choice overall though. Just don't send it out against Bruno Though.
Hope this helps!