I'm a fan of electronic shutters: If they are working, they are probably working properly, whereas a clockwork shutter is more likely to suffer from gummed-up, dried-up lube, etc. Despite what many people say, a problem with an electronic shutter may well be down to something that can be repaired (i.e. a solder connection that has worked loose). That said, if it does need parts, it'll rely on donor cameras of equal age. I do think mechanical cameras have a bit of and advantage in repairability.
Also, electronic cameras without an automatic mode can't take advantage of one of the electronic shutter's best aspects, which is the ability to set any speed between the detents.
All that said: I would consider the reality of shooting with a Mamiya 67. I have an RB67 on long-term loan from a friend, and I think I've only put two or three rolls through it. It's RIDICULOUSLY heavy (something like 6 lbs with a lens and film back) and has a very complicated system of interlocks. I can't tell you how many times I've sat there, perplexed, trying to figure out what I forgot to do that is causing the shutter not to fire. (Shutter/mirror cocked? Film slide removed? Shutter lock off?) Incidentally, one of the things *not* required for the shutter to fire is advancing the film, so double-exposures are frightfully easy. If you like spontaneous shooting, the RB is not your best choice.
Same friend also loaned me a Mamiya C330 TLR -- MUCH easier to use, and more portable, but still massive. Also, after years of shooting with a Pentax KX, the tiny "click" of the 330's leaf shutter just feels wrong to me. (To be fair to the RB67 does make the BEST camera noises ever. Voitt---KA-THWACK! Truly Don Martin-esque.)
Another friend (yes, I know, I have awesome friends!) sent me a Mamiya 645, and it's the MF camera I like the best -- much more portable, much easier to use, and the negatives still have that MF smoothness that 35mm lacks. If you aren't married to getting the biggest negative possible (and if IQ is your concern, why not just shoot digital?), that's the Mamiya I'd consider.
If the urge is just to own an RB/RZ67... well, I can't argue with that. I have to say, if you like mechanical things, the RB67 is a very satisfying piece of equipment. Fun to assemble, disassemble, play with and listen to. Taking snapshots, however, is not its strong suit.
Aaron