Which game has a better secondary / mercenaries type game in terms of both longevity and fun factor?
This is a very good question, and here's my opinion having put 50 hours into both Raid Modes in Revelations 1 & 2.
Revelations 1 has the better Raid Mode. Revelations 2's Raid Mode has some GREAT improvements to Rev 1's Raid Mode, such as new and creative enemy buffs, a more expansive and addicting character upgrade system, and various fun touches like a 'hub lobby' you can mess around in when choosing the next level and some pretty fun timed community challenges. However, Raid Mode in Revelations 1 wins in the end because of the following:
-The levels are better and have more variety/better design to them (the Raid Mode levels in Revelations 1 feel like they had more effort put into them and include interesting twists and turns I won't spoil, and it's also worth mentioning for some reason in Revelations 2 almost all of the Raid Mode stages are areas from RE6. Now I don't have anything against RE6, but since most the levels are ported over they don't get to do as much creatively fun with them as the first Revelations).
-The difficulties are better (In both there's multiple difficulties per level, and Revelations 1 handles this MUCH better than Revelations 2. Rev 2 basically mostly superficially increases challenge, while Revelations 1 does a much better job throwing curveballs on higher difficulty level stuff to keep things interesting).
-The end game is much, MUCH better (this was a BIG killer for me in Revelations 2. There's a 'climax' reward in both. Revelations 2 has a 'Code Red" mode you unlock which just adds harder versions of all the levels you've already played several times without much new to them. Revelations 1 reward however is an open world version of the ship from the main game with multiple choose-your-path routes, multiple endings points, a hidden exclusive to this stage boss at the deepest part of the ship to work towards. It's a very fun challenge, and the new port even added a Hard Mode version of this stage. This made it a fun goal to work at, while in Rev 2 the reward just feels empty and not worth it).
My opinion, Revelations 2's Raid Mode has some great new additions in terms of enemy variety/buffs and character upgrades, I think the enemy selection is more interesting in general and the new buffs make them fun and it's more fun working out a build for your character(s) in Revelations 2, but Revelations 1 wins for having better levels with better design and variety, better replayability, and a much, much better endgame reward which is much, MUCH more fun to work at than Rev 2's.