For reliability, I would go with hard wire over cord and plug. I think receptacles are much more prone to failure than permanent connections . Now, if you plan on having to move or remove them, a receptacle would be the preferred method.
Being in a barn I would coat the plug and receptacle with a dielectric grease, but other then a moisture problem I don't see 20 amp twist-loc failing under the small amount of current that these fixtures pull, and it's much nicer to be able to go up and unplug and un hook to bring the fixture down to replace the ballast in them then to try up at the ceiling, it also makes installation a breeze, I see the OP is from Wi. which means he's right around the corner from RUUD or Econolight, they will whip or pigtail the fixture for you if you order them that way and is much cheaper then doing it yourself.
I did a trucking company's repair garage using all mirror reflector 6x t-5ho's and they like them so much they had me replace all the MH's in their old garage, they told me they had so much light without shadowing they no longer needed drop cords for most of their work, that was 8 years ago and I think I replaced 6 lamps so far and 2 ballast, these were all RUUD fixtures, which with no middle man saves some money, Econolight is the same way but a little less quality but your paying for it, RUUD and Econolight are in the same building Cree purchased both company's a while back but so far Cree has kept the same way they run the sales, so you can order direct from them, I'm in NW Indiana and I can get my order within the next day, and they bend over backwards to make sure everything is right.
RUUD's link
Econolight's link