I had this same dilemma and ended up settling on the Aqueon Pro especially after finding out they are made in the exact same plant as the old stealth heaters and are the exact same thing with the addition of a power indicator light which was my main gripe about the old stealths. I couldn't find many bad reviews of the M series other than people said they don't like how shiny it is and wished it was black which is really a personal preference thing. I haven't setup my 90g yet so haven't got to try out the 250watt I ordered but the 50watt ones on my 10g and 20g are working great hold the temp +/- .5. Dials were both off by about a degree or so but o'well I've had much worse.
I'm leaning more towards the Aqueon Pro series right now. Only reason changing my heaters is b/c the Marineland heaters got recalled and these two are my choices. Got tired of shocking myself and cooking the fish with the stealth heaters.
I run an Aqueon Pro 200 watt. Have had zero problems with it and keeps temperature steady at 79. It is off by about a degree (my dial reads 80), but that is just a calibration thing. I like the black color too.