I have SAE wrenches, but basically just one full set of regular length combo wrenches, and one set of stubbies. Metric wrenches, on the other hand, I have combo, stubby, ratcheting, flex head ratcheting, XL straight beam ratcheting, and a large size set.
If you're working on cars made in the last 20 or so years then there will be very little SAE. It's definitely smart to have the basics in SAE for when you need it, but I wouldn't necessarily go overboard with high end wrenches like the X-Frame's unless you really work on SAE stuff regularly (or have money burning a hole in your pocket).
90% of the Snap On and other tool truck branded tools I see for sale in local classifieds is SAE, because some numb-skull noob tech spent thousands on high end SAE tools that they never use. And of course they always want to recoup 80%+ of what they paid the truck for them.