For this route, there are basically three choices:

AA, UA, and DL all have fares in the 400-450 range, round trip. All three award miles based on fare, not distance.
Alaska awards mileage based on distance, but their prices are much higher.

AA, UA, and DL will earn approximately 5 miles per fare dollar (base fare only), so probably 80000 miles if the employee is traveling 50 weeks per year.

Butt-in-seat miles (which is what counts for status) might be 200-250 per year.

If an average redemption round trip is 25k miles, any of these three would cover two of them, approximately.

Is this one single employee traveling every week, or sending "an employee" every week, which could be anyone? If it's multiple people, then it really doesn't matter which program, as the miles will be split between the employees. The employer does not get to keep or use the miles - they belong to the individual flyer.

If it is one single employee traveling every week, I'd ask their preference - they're likely to earn status halfway through the year, and may wish to use it the next year too.

In that case, I'd consider having them join two of the three programs, and collections miles and status miles on both if they are truly traveling every weekend - 200-250k miles is enough to get top tier status in two programs, so might as well get what they can out of it.

If they aren't flying almost every week, then maybe join just one FF program.

This seems to be a route with much competition, and three very similar mileage programs in terms of earnings and status requirements, so you can probably base your ticket choice purchases on service and preference.

Your employee should join all three programs, and take advantage of as many miles and as much status as possible.