One factor you can consider is how fast you recover from races. If you plan to race the tune up all out, then if you typically take a lot of time to recover from races, maybe the race 3 weeks out lets you take more time to bounce back.

You didn't say much about your training, how long you've been running, how many and how long your long runs are, and what your weekly mileage is. The more time and more miles you've been running, probably the less difference it will make if you substitute a tune-up race for the long run. I don't have any proof, but I feel that a hard race is equivalent to a long run about 1.5 to 2 times the distance of the race as far as my training goes, so I don't sweat running less mileage due to a race.

I don't think there is a blanket answer that covers all situations. Personally I would go with the 10-miler 2 weeks out, providing that my legs weren't trashed from over training or running too many races. Also, I think that tapering is often overdone. Five easy days with one of those being a day off would be plenty for me.