You can't go wrong with an MVP. There is no question that E.I. (makers of the MVP-50) stuff is the highest quality that money can buy. While the others are great engine monitors, the sensors mfgr'd and provided by the MVP are top notch.....you'll also note that AFS used and uses some EI sensors with their package as well. You'll notice that products from both of these companies are more than competitors, but some of what you're paying for is hermetically sealed machined sensors that are really works of art and cost a lot more than the standard off the shelf steward warning/auto stuff that lots of others use (not saying those are bad beause they aren't - just pointing out where the additional cost comes from).
Overall, I'd listen to what Jason and Gary at Aerotronics have to say because they've forgotten more about avionics and engine monitors than most of us will ever know.....they know their stuff.
Anyway, I also know the people themselves at EI are 1st class all the way - just like Rob and the guys at AFS.
Just my 2 cents as usual.
Cheers,
Stein.