I have driven them all at one time or another. For me anyway, it is hard to pick a favorite. The 120 is the purest, closest to Sir Williams idea. The 140 is "corrected" for the American Market. The 150 is just evolved more, but surely not as pretty. It will stop better though.
I LOVE the 120 FHCs. Given the choice of the 2 presented, I like the M model, with the wire wheels, no spats, etc, but that is strictly personal taste. The 120 drives great in the context of it's day. The steering is heavy by modern standards, and the brakes are not ...uhm, stellar, but they work fine in normal driving. The exhaust note has never been bettered. Working the Moss gearbox is a learned skill, but once mastered, they really are not so trickey to use. Finally, nothing stops traffic, or drops jaws like a 120. I once caused an accident ( thankfully did not involve me) just waiting in a turn lane to pull into a parking lot with a 120 roadster.
I have a customer that has a beautiful black 120 roadster. The car is largely unrestored, and he drives it A LOT, including day trips of a couple hundred miles with no problems. Like all new purchases there will likely be a period of "fettleing" to work out the bugs, but in general it should be reliable and easy enough to maintain.