the first upgrades i'd do on those are fresh tires/tubes and relace new rims from steel to alloy ones those alone would totally improve the ride.
have you ridden them enough yet where you could give us feedback on what you think about them? the Brown one with bigger wheels is the one I'd suggest you ride. It will probably feel more like a full size bike the the green one with a smaller/fatter wheel.
although both of them have 20" tyres they are not interchangeable one is 406mm bead diameter[green one] and the brown one 451mm. Its called ISO tyre size description is more accurate as most bike sales staff don't know and gets confused what you want when you say I need a 20" tire but its bigger diameter.
and please: don't repaint them ! just an advice
hope you have fun riding them steel frames they're pretty much bomb proof compared to the aluminium junk of most bikes today.