Your contractor is used to doing things one way and having done it successfully for a long time, so he thinks, "why should I change"? Essentially, he doesn't want to learn new tricks...or, he hasn't done his installs properly. If his installs are only lasting 3 yrs don't walk...run away! Mini splits are the only category of HVAC products that grew through the collapse of building beginning in '08. The projections are that in 10 yrs, 25% of all installs will be mshp's. Recognizing this, all the American unitary manuf are trying to get into it, but they are way way behind technically. Stick with the major brands for support purposes. Mitsubishi, Fujitsu and Daikin are 1, 2 & 3 respectively. They have parts and tech support. In looking at Fujitsu, which I own 3 of in my home, the warranty varies by model: 2 yrs parts/6 yrs compressor or 5 yr parts/7yrs compressor. In looking through the brochure there are only a few 2/6 systems left. MSHP's go in pretty quickly, but you need to do the process correctly. nitrogen purge/pressure test and evacuate with fresh clean vacuum pump oil. Perhaps I'm a tad **** about it but I change the oil every time I do an install. Power supply HAS to be clean. From an electronic standpoint You get surging/fluctuating power and nothing electronic likes it.
I represented Fujitsu in the New England market from '98-'11. I sold a bazillion of these things and I can tell you those Yankees would have cut my you know whats off if they only lasted 3 yrs. They are excellent when you have the right guy on the end of the wrench. All of mine are wall mounted on brackets made for mini-splits. The condensers stay cleaner that way.
In your case since you do not say where you are from it is difficult to advise on sizing. Minot or Miami? It does make a difference. For a building of that lay-out, assuming no petitions I would put an evaporator on each of the long walls diagonal to each other. You could do a dual unit or two individual units. Price it both ways. The dual may come in cheaper but you will have a bunch more money in line-sets, probably. My 24 dual with 2 12's is 16.5 seer (cooling) and 9 hspf (heat). My 12 RLS is 25 seer and 12 hspf. Your decision will likely be based on how much you intend to run them, assuming you can either get your dealer to get his head out of the sand or find a guy who likes to install these things. BTW, you might point out to this fellow that the factories offer FREE technical training on installation and service for installing contractors.
