Up to a point, you do get what you pay for, but that point varies, and isn't always true! How's that for waffle words!?
Many of the US manufacturers came up with a decent internal design, and have stuck with it for a long time - this allows them to use the economy of scale when adjusting the trim and handles to satisfy multiple styles and whims - your internal replacement parts can be limited to a few different items. Not normally true with Kohler. This doesn't make it a bad product by any means, just that when the time does come for a maintenace part replacement, because they change things fairly often, most places can't or won't stock all of the variations and it must be ordered.
Asking which is 'better' is sort of like asking which car brand and model do you like...everyone has their own opinion. One thing to keep in mind, the federal regulations regarding heavy metals (primarily lead) mean that the old test of weight often no longer applies - eg. solid brass verses plastic - the regs make it almost impossible to make most parts in brass today, at least those parts that water passes through.
Most people don't need replacement parts often, but it can take a long time to obtain them if you have something esoteric or unusual that isn't sold in big quantities. This may or may not be a big issue to you.
You should also keep in mind that most manufactuers have multiple lines, and what you may find on their economy line, while it may perform fine, could wear out or start to look funky earlier than those nearer their higher end stuff that often comes with a lifetime warranty on the finish and often the guts as well.
As a consumer, I've had decent luck with Delta and Grohe. Getting common parts for Grohe is fairly easy, but it is time consuming on replacing things that don't normally break. I had my WH replaced, and it stirred up enough crud to stick the check valves in my shower valve. It will likely be over a month. It runs, but produces a feable shower - enough to get by on, but not its original self. I could probably take them out and clean them, but in the chance that doesn't work, and I only want to take it apart once, I ordered new ones. On the other hand, I can stop in the local supply, and pick up a cartridge today. Some people swear by Price Fister, Moen, etc. Just don't pick their economy stuff if you want it to look good long down the road. Some companies don't make an economy line.