I went from a Minolta Autocord TLR to a Bronica ETRSi, which is 645 SLR, similar to the Mamiya 645. I prefer the 645 aspect ratio. I have both a waist level finder and a prism. For street photos, its unobtrusive to use the waist, but I never do because I like to do verticals, and composing even horizontals quickly with the waist is difficult for me with the image reversed. With the prism on, the camera looks as intimidating to strangers as any big SLR. Another thing no one has mentioned is mirror slap on a med format SLR. It is difficult to hand hold for lower shutter speeds compared to the TLR. I think you are better off for what you want with your TLR.