Scenic route from Santa Fe to Farmington, NM

Your first option, 84/285/64 would be more scenic. US 550 would probably be faster as it skirts by the mountains. Even though 84/285/64 goes through the mountains, there was nothing that created any difficulty.
We took 84/64 last summer from Santa Fe on our way to Durango and enjoyed the drive. Our last trip over 550 was a few years ago when much of it was under construction. There really wasn't much to see along that road.