I am going to talk crazy because I have a Volvo 740 turbo and two Saab 900s. Do note that I personally do not like the 240s. And all this is IHMO:
Volvo 240 ===============================
PROS
o Easy to work on though the 700/900s are even easier.
o Cheap and abundant parts. Many of a 700/900 will fit it. Well, some parts of a 140 will fit it.
o Was originally designed to take, as one of the options, a V8 engine. The project end up losing two cylinders and became the PRV engine. Which is why people put different engines in it, from the usual SBC to a VW TDI engine.
o Rugged. It is used in many different kinds of races. Also, it takes more than nasty words to compromise its cabin.
o Engines can be built to reliably deliver 600HP. More if it is a race-only car.
o RWD.
o Makes it easier to swap transmissions. Many people have put T5s, W58s, and Getrag 265s behind those engines.
o Good aftermarket support (at least in Europe but some in the US).
o Volvo wiring diagrams are one of the best.
CONs
o Use the worst fuses known to man
o Some used Chrysler ignition that goes on strike if you look at it wrong.
o The transmissions could be better. The Manuals have horrible ratios.
o It is ugly IMHO. But, many like it for its looks and say the 700s look too American. So, it is a matter of taste.
o Relays were laid wherever they felt doing so. In the 700s, they were together with fuses in the center console so you could reach them from the driver's seat in the rain.
o Hood does not go vertical like a 700.
o Biodegradable wiring insulation until late 80s.
Saab 900 =========================================
PROS
o Very unique lines. Once again this is subjective: some like others hate it.
o Easy to replace clutch.
o Very though body. I know of someone who rolled down two levels in a mountain on one and only cracked the windshield.
o Very good on snow
o Engine can be built to deliver more than 400HP reliably
o saabcentral. Honestly that is one of the main reasons I have one. The help and support here, specially in the Classic 900 area, is nothing but amazing.
CONS
o Transmission, both manual and automagic
o Changing suspension parts is not fun. Not the shocks but the bushings.
o Not as much space in engine bay as a Volvo
o Small selection of engines that fit in it
o Why are the fenders/wings welded to the car?
o Uses the same awful connectors as Volvo does