Lets say you were shooting the bow with the 21" riser. and you put limbs that were 20-1/2" long. you'd end up witha bow that was 62" long. you put the same limbs on a 17" riser, and you have a bow that is 58" long. Put an arrow in each, and 62" bow shoud feel smoother, as it is longer, and less finger pinch. I like a longer bow, some like shorter bows. Given the choice you are asking, I'd take 17" riser with the longer limbs = 62" Should not stack with longer limbs. I'm shooting a 60" bow, and wish it was a 62" Personal prefrence, but longer limbs in my opinion are usually smoother.