12/2 for 20/1 circuits, 14/2 for 15/1 circuits. The term “MC” is for metal clad, which if your running exposed would be OK as long as it fits the NEC’s (National Electrical Code) permitted uses, which I think does. {my preference for surface or exposed installs would be in EMT/conduit}
Make sure you get the boxes and connectors right as you’ll have a better, safer install using all the right parts.
Note: the guy working the electrical department at HD isn’t the best reference in many cases where there are questions on Electrical installs, get a DIY or reference book where someone has gone on record with that install advice.
Either that or obtain the services of one of the many skilled Electricians across the country. Good luck.