I bought the 645 Pro TL recently and am very happy. It is certainly more portable (& much lighter) than my 5x4 camera. As I tend to use a 6x9 rollfilm back with my LF camera I haven't bothered to date to go for a 6x7. One possible alternative, which resolves the weight-format equation, would be to buy a secondhand Mamiya 7 / 7II camera. I know a professional landscape photographer in London who swears by the Mamiya 7. I hope to buy one and its 43mm lens (the 35mm equivalement of a 21mm lens).

Reviews of Mamiya & series cameras:
http://www.kenrockwell.com/mamiya/7.htm
http://www.luminous-landscape.com/re...iya_7_ii.shtml