You can't go wrong either way.
A Basin skied very nice Saturday and Copper skied great today with more soft snow than I expected and no crowds. I haven't been to Keystone in a few weeks and haven't been to Loveland this season. A-Basin wasn't overly crowded Saturday, but that may have been partially due to the cold temps.
While the resorts are only about half open so far (and no good storms this week), you should be in for a very good week with reasonable temps and nice snow conditions.
Given that you have a AB-KS 4 pack, I would start off with Keystone and then hit A-B on your 2nd day. I say this because of the lower elevation at KS compared to A-B.
From Frisco, Copper is the closest resort and the mid week crowds there have been super light. Like Mike said, it is a very good mountain and not small (even with only half open). The free parking is closer at LL but you can take the bus to Copper pretty easily from Frisco (and I find this to be more convenient than the free parking).
Mike_M & I both instruct at Copper and they are offering 50% off private lessons through December 18 with $50 lift tix available for anyone in a private (up to 6 friends and family). I didn't mention this in my PM to you, but if you do book a private at any resort and have an instructor in mind that you want to take a lesson from, it is always nice to contact that instructor directly or make it clear when you are booking that you are requesting a particular instructor as most places (including Keystone and Copper) pay the instructors a higher rate for request privates rather than "assigned" privates. There are also 2 for 1 coupons for lift tickets for Copper.- I believe the ones I have are good starting Dec 14...I think these are still on sale for $10 as part of a Frisco Elementary School Fund Raiser.
LL crowds can also be pretty light during the week. If you like the non-resort feel of A Basin more than Keystone, then consider giving LL a try. Like A-B, LL has no lodging. A-B has 1 detachable high speed lift and 2 places to eat while LL has no high speed lifts and 1 place to eat.
If you prefer KS over A-B, then go to Copper on the extra day- 4 high speed lifts currently turning and closer to a dozen places to eat at Copper.
For a good restaurant in Frisco, go to Vinny's on Main St b/t 3rd and 4th. Sit on the bar side if you want a more moderately priced menu (and the option to order off the dining room menu with more upscale items.
For on mountain dining at Keystone, I prefer North Peak over the cafeteria on the top of Dercum.