RE: More Important?
Yes. If I remember correctly (I haven't bought any in awhile), the rare earth magnets were about 1/8" in diameter.
If you are concerned about the thickness of the fuselage side, just CA some scrap balsa to the inside of the fuselage where you plane to put the magnet.
The way I do it is to make a hole where I plan to put the magnet, then "attach" the magnet to the flat side of a 11 blade. When I am happy with the fit, I put the CA in the hole, insert the magnet and "wipe off" the 11 blade against the side of the fuselage to leave the magnet in the hole.
Don't forget to inlet the steel plates so they sit flush. FWIW - I used some scrap sheet metal (24 gauge ?) I had gotten at Home Depot and cut strips 1/4" wide x 1/2" long using the cutoff wheel on my Dremel.