If memory serves, the Pentax 645N weight is 45 ounces for the body without batteries. (Because it takes only 6AAs it's very close to the exact same weight as my Nikon F5 at 42 oz that takes 8 batteries)

The Pentax 645N II is no longer being manufactured (but neither are Contaxes. Pentax is still in business, however).

The much more readily and cheaply available 645N lacks only mirror lockup and a few other subtle features via firmware customization like additional data imprinting choices on the edge of the film, and 15 shots on 120 film instead of 16 (the 15-shot function seems to me more about fitting 3 row of 5 neg pages, than due to any widespread film flatness issues, though there is one guy who mentioned this and it gets repeated endlessly via the web).
One remaining less subtle feature might be 1/2 stops on shutter in Manual metering mode of the P645NII (note that shutter speeds are stepless in A priority and Program, anyway).
A key point about all Pentax 645s is that they really don't need MLU, because the mirror is so well braked (unlike the 67's-- for which it could be crippling). My recommendation is to save yourself an easy $4-500 by buying a mint used 645N if considering Pentax. The other bargain hint is that if AF doesn't matter to you, the older but exquisitely well built manual focusing Pentax 645 SMC-A lenses can be tremendous bargains. Some like the macro 120mm have the same optical design as later AF versions. I don't find much to fault about my tack-sharp 35mm f/3/5 SMC-A which I bought for under $300. It focuses to one foot. The optics of the FA 35mm are reputedly slightly improved, perhaps more obvious with digital.