Personally I've preferred the sound of every YUS5 to every YUS3 or U3 that I have played, and I'm not quite sure why that is.
If the YUS3 and 5 are of identical scale design then it must have been down to the preparation of the models I played.
Basically if you're choosing between two pianos that are virtually identical, then go for the one that you prefer, because that is the one you'll have to live with. They're both pretty good pianos and I don't think you'd feel let down by either one.
They're both the same height, the 5 isn't taller than the 3.
While you're at it, you could have a look at the SE series pianos, and the SU7, but the SU7 is far more expensive. I would have a look at them just for fun, because I like to find out what's available.