If you have been lurking long enough you will have probably seen my dislike of round rail/bearings and complete louthing of unsupported rail.!! . . IMO really is no comparison between round rail and profile rail.! Profile rail makes the difference between DIY machine and Pro machine that will out last the machine and out perform round rail by magnitude of 10X.

To me if your serious enough to invest couple of grand and several months of hard graft then the extra cost is just not worth skimping on and like Jonathan says if you've waited 5yrs then don't spoil the broth for a pinch of salt and wait untill you can afford them.

Regards size then 15mm will more than handle the load but I find 20mm is the better option due to bearing size being slightly larger offering better support and easier mounting. 25mm is just over kill unless really large/heavy machine and waste of money plus has well has being inefficeint and stressing the motors more.