Jim, in my most humble opinion the thing thats going to make the difference is how fast your trying to make your cart to run. With either the 18 or the 23 your cart should run the same speed (mph) on flat ground. It's pretty much set in stone, (engine rpm--gearing ratio--tire size), will produce the same results no matter what which of these engines you have.... Now if your wanting to change gears and/or run some ridiculously big tires, the 23 will be better at, getting to and maintaining top speed, especially when going up big long hills. Also I have allways heard that the 23 has hp intakes setups available that the 18 doesn't have, so the 23 is better for modding. I know you would be happy with the 18, (it would take weeks to get the smile off your face) but if you want to get to that next level, i'd go with the 23 and mod it up. You could be blowing away rhino's in your ezgo workhorse
If I ever manage to blow up the engine in my cart, (i'm trying) it will be replaced by a 23 or bigger.