I switched from Kiev 88 to Mamiya 7, and honestly regretted it. The Mamiya 7 glass is super-sharp, but it lacks character. As others have pointed out the Mamiya isn't good for close-ups, either, and only has a few lenses. That said, the Kiev broke too often - the importer was nearby and fixed it while one waited, but it still got frustrating.
After a few months I sold the 7 and got a Pentax 67. Nowadays I'd get a Mamiya RB67. The older ones are only a bit more expensive than the P6, and it's likely to be more reliable. You also have a choice of cheap lenses - the "C" series, or the K/L stuff, which is really good, and only a few dollars more.