I dunno, more powerful motor on the the L, speed actually slightly slower than the M which is a little known fact. The L states a pumping distance of 20metre, 15 for the M, although I have pumped upto 25metres with an M and at height. So yea, for my money out the two the M's the winner, smaller, lighter machine that does what it's big brother does but a little bit faster. The slimline though, that's another kettle of fish.