I’d go ahead and make the larger size and if needed, fold the cuff’s up more than normal.
My daughter has unique fitting needs (especially the tops—length and waist) due to an birth defect she has, so I’m always in here buggering people about sizing!
My daughter is about to turn 3, and has been consistently wearing size 2-3 in Oliver + S (I usually have to lengthen things, but they fit well). I made size 2T in Late Feb/Early March. She went through a HUGE growth spurt, and I’ve noticed that we’re almost to the point of needing to unfold the hems. But otherwise, she has PLENTY of room still in both the pants and top.
I would think that a kid old enough to wear size 5, isn’t growing nearly as fast as a 3 year old (since growth slows down a bit around this age), you’ll be fine with whatever you choose.