If you want to play RDR1, you should do it before RDR2, not because of the story. But the game will feel small, and lesser next to RDR2.
Personally I’m not terribly concerned. RDR1 will feel lesser, yes, but it’s still big enough of an undertaking that it would have delayed my being able to start RDR2 very substantially. Better to have RDR2 at the head of the queue while it’s still new-ish, I think. It’s one of those things where it’s best to experience the cultural phenomenon as and when it’s happening (even if the game will probably age well, and be a classic forevermore). In the meantime I can let RDR1 accumulate a bit more “vintage” status, so that when I finally get around to play it, it’ll feel more like a slice of video gaming history, which has a worth of its own. The stage at which I’d feel least inclined to play RDR2 is when it’s too old to still be “the hotness”, but not old enough to be “vintage” just yet.