I was going to buy a SanDisk Extreme Plus, but then I noticed the Samsung Evo Plus has the same read speeds but less write, at a significant price saving. Write speeds are much less important so I'm going to buy a 128GB model I think.
I'm tempted by a 64GB version because I don't play on buying Switch games on the eShop as I like being able to sell games, buy used and take advantage of sales. I think 64GB will be enough for a quite a lot of indies & Gamecube virtual console games (if that really happens).
Though maybe I should indeed get the 128GB because I like having Mario Kart and Animal Crossing digitally as I jump in and out of them a lot, so not needing to load a game cart would be convenient like it was on the 3DS.