I'm running two gyre 230's, so I would disagree with your LFS. Each 230 goes from 10% to 40%, so I have room to increase at some point if needed. I had an MP10 on the reefer when I first set it up, I wasn't comfortable with the wet side and dry side magnets on the glass. Others have had success with the MP10 on the reefer tanks, so it is possible. I ran an MP40 on my reefer for awhile and I like the pump, I just feel like the gyre is a better pump for the short 3' tank. If you look at the rating the MP40 pushes more water than the gyre 230. I think your LFS is confused or they like the MP40's for personal reasons, and it's possible they want to sale you the MP40 because it is much more expensive.
You can have success with any pumps you decide. For me the wife flow coming out of the gyre on the short tank wins. You could probably get away with one gyre 230. It is also cheaper to buy the gyre pump with controller and then a second pump VS 1 MP40.