No Regret was the harder game but I think the available weapons had something to do with that. In No Remorse, enemies started out with weaker weapons like the BA-40's (handguns), In No Regret, the weakest weapon was the RP-32, which was one of the higher tier guns in No Remorse. In No Regret, if you got hit with anything other than the RP-32's, you were in for a very serious owwie. Remember in No Remorse how later in the game, some enemies had one hit kill weapons? We'll they started showing up a lot sooner in No Regret.
Also, in No Regret, the devs were even more punishing about hiding those keycards. Sometimes there was more than one way to complete a level (which could be easier or harder). I remember in one level, you could blow the floor open, drop down and take an alternate path to complete the level. I forget which level this was though.
I also prefered No Remorse to No Regret because of the rebel base interludes. Talking with your fellow resistance members and the "news" sequences really added atmosphere to the game. I prefered Trina Jenkins in No Remorse because when she delivered her lines, it was cheesey as hell but it fit the atmosphere perfectly. She was a lot more toned down in No Regret which made the sparse "news" segments not as fun to watch.
I beat both games on loose cannon difficulty. After clearing a few games from my backlog, I'll have to go back and try playing through on the hardest setting :)
i remember i interrupted No Regret at level 4 or so, then i dunno why i didnt play further i think i was a bit pissed my lovely game was so unfair somehow and i didnt want to lower the difficulty ;), i will retry soon!
but i also had the feeling the game was a bit rushed released cause of the lack of the interludes etc.