as previously stated, RDR is lot easier. first time thru, I only failed few dead eye hostage situations in rdr, whereas in GTAIV I had to retry various car chase missions and ally protect missions dozens of times.
Its worth noting that the challenge in GTAIV was mostly due to weak game mechanics and sometimes unbalanced missions. In RDR, game mechanics were more polished, and missions were much more balanced and well thought out.
Compare GTAIV to V. In V, game mechanics were super polished, which made the game really easy, the easiest Rockstar game to date.
They both have the same game mechanics and auto aim. I think they were around the same difficulty tbh which is kind of too easy. Sometimes to make things interesting in games like these I only use pistols. I also play Uncharted games like that and go for all headshots lol
I think GTA IV is a bit more clunky and tougher to get the hang of at first. I remember there being some missions that were a pain because you couldn't let friendly AI die. It's been really long since I've played RDR, but I want to say it was easier since the aiming was more precise and you could use Dead Eye. Also, if I remember right, RDR has checkpoints in its missions and GTA IV does not.
Good responses here. I guess now that I know how to play these kinds of games better from all my practice with GTA4, I am less liable to make mistakes in RDR. But it also helps that every time I have to redo a mission in RDR, I am not sent back to a point where I have to drive through a giant city for five minutes just to get back to where I was in the mission! RDR seems much more forgiving in this regard. I guess its save system is different.