Technically, you are incorrect. Loki Research has a 54mm M motor. Possibly too aggressive for what you are trying to achieve.
Regardless, it really depends on what you want to do. If you want to have variety to fly different propellants and different size motors, RMS is the way to go. The Aerotech line has a Reload Adapter System for their 75mm casings, allowing you to go down 2 grain sizes. So if you have a 75/6400, you can fly 75/3840 motors. Having the 75/6400 grants you access to 19 different reloads. Adrian wasn't kidding about being the swiss army knife of reload casings.
I had the 75/6400 before someone lost it, and bought me a replacement case (I paid the difference and upgraded to the 75/7680 case since I already have a 75/3840 case, and with the rockets I fly the 75/3840 in, the extra motor casing would be a waste of space. I love the Aerotech 75mm line. The M1315W is a favorite of mine, though I got my L3 on the M1297. Either way, great motors.
Alternatively, if you just want to get your L3 and dip your toes in the larger diameter and only see yourself flying once or twice, the M1350 might be a better choice. I regularly fly big motors, so the RMS was a good investment for me. The RAS also allows me access to more motors, as I indicated above. With the 75/7680 and the 75/3840, I can fly the entire range of Aerotech's RMS reload line.
Adrian is also right. Get the 54/1706 with the RAS. I was just moving some boxes around and realized I have no less than 4 54/1706 motors and I'm pretty sure I have 2-3 more in storage. I have got to stop finding folks getting out of the hobby on accident (no joke, these folks are attracted to me! Guess it shows how long I've been in the hobby...)