Stan,
I recently bought a 5100, so I can't speak to its reliability, but my 401 and 540, have been run for a couple of years with zero problems. Maybe that says something about Dolmar's culture regarding product reliability.
Both stock, its pretty close, if not equal to my 359, don't think you could say that about any other 50cc saw. It does weigh a little more than the 346/353, and might suprise you how close it "feels" to a 359 in weight and "volume" (don't know if thats ever been used as a subjective comparison point!)
If you are really sensitive to compactness and handling, go 346, if you'd give up a little of that for power, then the 5100. Or as one of the banned builders told me, if you want a 5100, buy a 359 get it modded and end up with something about the size/weight of a 5100 that will run with a 372! Something I will be verifying.
Luke