The viewfinder is clear. Not Leica-clear (at least not M2/3 clear), but very awesome nonetheless.
Why are you going with 7 and not the 6? The 6 is much smaller because the lenses collapse, comes in black (or at least isn't so rare as a black Mamiya 7) and doesn't need an external viewfinder. Apart from the fact you get the 43mm (which is a 20mm 35mm equiv.), 65mm and 210mm lenses for the 7 I personally don't see much reason for the 7. I would never shoot anything wider than 24mm (50mm on the Mamiya 6). For me, if I'd ever get either, I'd definitely get the Mamiya 6, simply because it's so much smaller. Either way, I'm sure you'd be very happy with either.