I've got an SQ-B and a Mamiya RB 67 Pro S and love 'em both for very different reasons. For street shooting, the Mamiya works best on a tripod or monopod. The Bronica is almost petite by comparison and works well handheld with a WLF or 45 degree prism finder. The manual speed winder grip really improves its ergonomics, too. Just be aware the SQ-B won't power Bronica metered prisms made for the A/Ai models. While Mamiya RB 120 backs are still fairly plentiful, 120 backs for the SQ series are hard to find and usually pricey. With only 10-12 shots per roll, a spare back or two is nice to have. Focusing on either with the WLF+magnifier is easy. The Bronica has a split image focus aid; the Mamiya screen is like a small plasma panel with the magnifier--bright and sharp.

Using a handheld meter is really the only way to go with these. A small handheld incident meter is really all you need--something like the Sekonic 318-328 or slightly pricier 308 are small, accurate and easy to use. Lugging around another camera just for its reflected meter will get old fast. You might want a spot meter later but an incident meter will get things rolling.

The medium format shooting style is slower and more deliberate, thanks largely to the gear itself. The jump in image quality over 35mm isn't subtle. You'll like it.