I live in Boulder and have an Epic Local pass, so I've skied a fair amount at both resorts. While A-Basin is considered more of a locals hangout spot and gets far fewer people, it is also a MUCH smaller resort. If you're looking for some serious steeps and want to challenge your skills, then that's absolutely where you should head. However, I've skied Keystone on some VERY busy weekends, and although certain runs will get crowded, there are ways to avoid the crowds.
1) Get there early. If you want to ski the Outback and not be crowded, be in the lift lines before 8:30, and head straight back. By 10-11am those back bowls will be packed.
2) Don't take the River Run Gondola up. Instead, cross the bridge and hop on the Summit Express. They both take you to the top of Dercum Mountain, but the Summit Express reliably has half the wait time.
3) When the mountain starts to fill up, the frontside blues will stay weirdly quiet, especially Wild Irishman and Paymaster. Schoolmarm will be PACKED (though there's a very short run called Hoodoo off of it that is always completely empty), and Spring Dipper will start to get quite busy (though Santa Fe is usually pretty quiet, if you stay to the right about halfway down). Frenchman and Flying Dutchman will also both get their fair share of traffic (though nowhere near as much as Spring Dipper), but I've always found Wild Irishman and Paymaster to be damn near empty.
4) Mozart is a NIGHTMARE when the crowds start pouring in, but there's a not-quite-secret black run called Diamond Back that has it's entrance at the bottom of the bunny slope, so it really doesn't get that much traffic. If there's been a fair amount of powder and no grooming, it can get pretty difficult and bumpy, but in general it's a nice, steep black run with an very straight even slope that can be really fun.
5) Last tip: If you ski down to Montezuma and you're approaching from the Schoolmarm/Last Chance side, ski past the lift line and enter the line from the Spring Dipper side. When the lines start to get long, you can easily shave off a few minutes, just by going around to the other side.
Anyways, hope this helps. Enjoy!