i dont think you can really truly compare them to each other... they are all different and are more "specific" for what you want out of a motor? know what i mean? it all depends what you want, then you can say "this motor is best for that"

if you want cheap and reliable there are tons and tons of windsor motors out there, and you can mate 5.0 mustang parts to them i beleive? so you can get cheap power also, theres more options for that motor also.. it's just as popular as the 5.0 so theres lots out there for aftermarket, as they are the same "family" of motor too...

i dont know much about the clevland but i beleive its a pretty good power house motor, and has lots of potential, but more expensive cause there isnt too many of them around?

the modified motor is more of a torque monster than anything, but if you were to go with an M motor go for the 400.. the 351m and the 400 are the same thing just the 400 is bigger, so again more torque. i have the 400 in my 4x4, with some work done to it and i can burn off the 35" tires with the stock 3.00 gears! but after 3,000 rpm or so she dies out... also the 351m/400 use the "big block" bolt pattern for transmition housing so keep that in mind too