As a SoCal native, I’m very familiar with both companies as they are both within about 30 miles of where I live. They are both good and very similar… a lot will depend on your local sales reps and support. Both companies build very nice barns that are low maintenance and lovely.
At my farm, I have a Frankenstein mix of pieces of pretty much every barn company that has existed in our area over the last 30 years except for Port-A-Stall! We have MD, Barnmaster, King, and a lot of FCP. There is not a lot of difference between similar vintages of barn parts, but the differences do become apparent after a decade of use, and that is in how the barn was designed and set up.
All these barns (and pipe corrals too) will eventually rust along the bottom where they are in contact with either damp bedding or the soil in the case off corrals. I would consider putting all the barn walls up on stem walls that elevate the panels at least 6" above the anticipated grade of the stall floor, maybe even more. That will greatly keep damp bedding away from the metal- but it will also put your walls kind of up in the air and you’ll need to use something like 2 x 6’s across the stall doorways to keep the bedding in.
When your barn walls rust at the bottom (not if) you can do a couple things to get another decade or two of life out of them. We several times took the walls out, flipped them bottom-to-top, and put them back in place, then used Bondo to patch the rusted spots, which were now almost 8’ up in the air doing no harm.
Metal barns in appropriate climates really do offer a lot of advantages over wood, but there are always drawbacks as well. Horse’s can’t chew the metal, but they can kick and ding it bad (we end up lining our stalls with 1/4" x 4’ high rolls of conveyor belting rubber which is magic stuff at a boarding stable!) and horses can also scrape their teeth on painted metal panels to make them look awful. And as one of our more young-mined horses proved again this week, yes they can kick and scrape and literally peel metal panels out of the frames and delaminate them if they want to bad enough. More conveyor rubber, STAT!
Spend a lot of time asking questions with different barn builders and you’ll hopefully end up with your best value and choice in a barn. The materials certainly matter, but design, installation, SITE PREP and after the sale service are also critical to your future happiness…and I certainly condone doing the barn porn thing on Pinterest to get tons of layout ideas before you start.
Good luck, happy barn building!