Mapei says neither should be used for joints smaller than 1/16", unfortunately, so we're looking at either the Keracolor U or the Polyblend Non-Sanded. I'm reading that people generally prefer the Mapei, though, so may be worth it to get that.

1/16" is the minimal allowable grout width per TCNA. Agree with Verbo on Mapei

Thanks! So would you use the Ultracolor Plus FA even with some joints that are shy of 1/16", or stick with the Mapei unsanded? We have full 1/16" joints (done with spacers) between the rows, but within the rows are using the built-in 1/32" lugs on our tiles--so nominally that's a 1/16" joint there too, but really it is less where the lugs meet, so may be tough to get grout into.

Actually, I would and do use Prism grout from HD. It will do what you want, is pre sealed, antimicrobial and the colour is true....

Thank you! We ended up using the Ultracolor Plus FA (just because we chose a Mapei color) and all went well. :)