They both have their strengths and weaknesses. Besides RDR is a western game and GTA is a modern game. These two differences make them incomparable besides both being sandbox games by Rockstar
Not at all
Gameplay is almost the same. The only difference is the setting of each game.
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I have 2 major things that make RDR irritating sometimes.
1) If you fall off a cliff on your horse, you both fall like a boulder and really fast. While you're on foot, you fall realistically. I'd really love it if the horse would do the same.
2) The map is really small, and with just 3-4 'towns' it can get pretty boring. I mean, sure, it's a really great map, but for such a vast environment, they could have added more. More caves, more gang hideouts (both big and small), more active camps, etc.