gosk8ing,
Ok, here are the differences between the two engines. The FS-70 has ball bearings in it where the FL-70 has bushings. The FS-70 has a low speed needle valve where the FL-70 has an air bleed low speed adjustment. And finally the FS-70 has a metal backplate and the FL-70 has a plastic backplate.

Of all of these differences I would say the biggest of them is the low speed adjustment. IMHO an air bleed low speed adjustment is about the most worthless thing on an airplane engine. They are very hard to make minor adjustments and very frustrating. I fly almost all OS engines and I have had exactly one air bleed adjustment, and will never have another one. Period. If you want my opinion spend a little extra and get the FS-70.

Hope this helps

Ken