Q: I know they came with two different carbs, is one better?
A: I thought they all had two HS6 SU carbs - at least in the US market. If referring to Strombergs I have no experience but from what I hear they are troublesome. SU's work quite well but tuning and balancing is a black art. I'm getting slightly better at it, however.

Q: Can they be converted to injection (from a 140?) w/o much trouble?
A: Depends on your definition of 'much trouble'. It isn't straight forward and unless you have a complete running FI parts car it would probably be prohibitively expensive. It involves installing a B20E or B20F motor, plumbing in the high pressure fuel pump (possibly a fuel return line unless you put the regulator back by the tank), installing the FI computer and complex wiring harness and getting it all working. Meanwhile FI parts keep climbing in price (D-Jet, in any case). I personally wouldn't bother with it - I think most of the HP increase of the B20E engine over the B20B is attributable to the higher compression ration and different cam - rather than the FI. However, the FI intake manifold probably adds a little mid-range oomph.

Q: Are there any advantages to the B20 over the B18 other than more power?
A: None that I can think of. Its really the same engine with slightly larger cylinder bores in the casting (you can't bore a B18 out to 2.2 liters, but you can do that to a B20 block). They do have a few more mounts in the block for alternators, A/C compressors, etc.

Q: Do they run as long as the 240's?
A: Yes. The main mechanical components will probably last just as well or better than a 240 as long as you don't let rust eat the body. However they are a slightly higher maintenance car. The 122 has drum brakes to adjust, grease fittings to grease, valves and timing to set that a 240 doesn't. Not a big thing if you do it yourself, could be expensive if you take it somewhere.