At 2 weeks out, you are splitting hair as to what is best. I personally would go 8 miles easy and 6-8 miles at marathon pace rather than 20 easy at 2 weeks out. The 5-8 miles at marathon pace would give you more data points about your fitness and marathon pace reality than then 20 easy. If it works out well, it would be a great confidence builder.

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My marathon PR is "under three, high twos. I had a two hour and fifty-something."

I think 17-18 miles is fine 2 weeks out versus 20. As far as quality goes, it depends on previous weeks, the following week of training, & what you feel like you need. If you're feeling tired with 2 weeks to go I would just run easy for 2ish hours. If you maybe got sick or missed a session or two something like 2 x 4-5 miles @ GMP in the long run is totally fine. I do that long run & then my only workout the following week is on Thursday and it's a gentle 12k progression, GMP >>> HMP. The following Tuesday is something like 4x1200 or 5x1k @ T or 5k @ GMP. Might throw in something like 3x2:00 in an easy run later in the week. Strides the day before the race after a short run. Basically what I'm saying is that the work is more/less done. Figure out if you need more quality or if you need to rest up & get the body right for race day.