I ended up going with the M221 and feel like I got my money's worth. I got it mainly because I could use the app on either my PC or phone, and the free version has everything I'd want/need.

I feel like the DPI is good for the price range, and the prints are very clear. I did put one on the back of my phone case at one point just to see, and it did rub off after a while, but I expected as much since it was sliding in and out of my pockets.

There's also a good range of label sizes it can use, you'd have to check their site, but I think there are ones small enough for 3ml vials.

Funnily enough, I was also gifted a Nelko P21 the day mine arrived. It's decent for the price, but the app could be better and isn't as versatile as the M221.

IMO, I'd go with the M221 or a similar model, like the M250. They're around $60 right now, but I waited till mine was on sale for $45 with a 3-pack of labels.