Yashica Mat 124G vs Mamiya c330 [light meter/exposure]

The light meter on the 124g is grand for the most part, I've been happy with it for B&W and C-41. I'm not so sure I'd trust it with slide. It does ideally need a mercury battery replacement of some description, I used to jimmy sr44s into it and just compensate using the iso dial.
Other than that the 124s are pretty nice cameras, and pretty well regarded.
The mamiya cameras are huge, solid, and heavy. they focus much more closely than the yashica TLRs and I think they have some way of displaying parallax errors, but closeup stuff is probably the last thing you want to be doing with a tlr anyhow.

Light meters are easy enough to use. with the modern digital meters you just use them in either shutter or aperture preferred modes. Ie, enter your desired shutter speed, take a reading, and you'll get the corresponding aperture value or vice versa. Typically then you can click up and down through the equivalent ev values. Older meters I've used you take an ev reading and then transfer it to a dial which gives you corresponding pairs of aperture and shutter speed values.

My 124 has a taking lens that maxes out at about 3.5, and a viewing lens at 2.8. Couple of reasons, one is to ensure that it's as bright as possible, the other gives you a bit of leeway woth focusing. If it's in focus at 2.8 then you can be reasonably sure it's in focus at 3.5. I guess they didn't regard the yashinon's performance at 2.8 to be acceptable for a taking lens but it's grand as a viewing lens.