Ragwing C-140 is more nimble in the air than a 150; more "immediate" control feel. But not "squirrely"- if it's rigged and trimmed right, a 2-seat Cessna will behave very nicely hands-off. The 140A (metal wings, single strut) are more similar in feel to the 150... after all, the first 150s were just 140As with square tail feathers and a different gear configuration (the wing and cabin section are virtually identical). But they are sweet flyers; definitely a little different. Off the top of my head, I'd guess the average 140A is probably a little lighter than the average 150, even the early square-tail 150s (assuming the same goodies in the panel, etc).
The C-120 is basically a "budget" version of the 140- no electrics (I think), and no D-windows, flaps, etc., although a lot of them now have D-windows, flaps, metal wings, bigger engines, or all of the above. I met a guy in San Diego who had one that was even set up for basic IFR. For night flights in that area or penetrating the marine layer, he found it more than adequate.
As entry-level taildraggers, I don't see any problem with them. Because of the fairly modest "deck angle" and the rounded glareshield, visibility while in 3-point is not bad. Plenty of rudder authority, and not prone to weathervane more than other types. A lot of them have extenders to bring the mains forward a few inches to prevent nosing over... whether that's necessary or not, especially for beginners, will depend on who you ask.
I've flown a few 140s, all without extenders if I remember correctly, and didn't have a problem... personally, I think the nose-over thing with 120s and 140s is mostly mythical; didn't seem nose-heavy to me at all. You can put almost any taildragger on its nose, if you abuse the brakes; the 2-seat Cessnas are no exception.
But what do I know? You will get a lot of useful info on them here:
http://www.cessna120-140.org/