Two things I've noticed between the MS250 and the MS251 are this. The MS251 is cold blooded probably because of being a cleaner, better mileage engine, takes just a bit longer to warm up unless you richen the L screw more than optimum tune. 2) the MS250 is lighter by .8 #s I think.
There is nothing wrong with a MS250, ran them, like them just like the improvements more on a MS251.
I like the 251 better as well.