I have both a 501CM and a 503 CX----Since I have the TTL flash metering on this 503 version and I have no desire for a motor drive I am well pleased----I have a 250 mm but no issues using it on the 503 CX (non Gliding Mirror) and therefore the 501 CM (with GMS) and 503 CX and interchangable with the exception of the TTL flash metering---I use a Metz 45 CL-4 on my Mamiya 7II (no TTL control on the M7 and dedicted TTL on the Hassy) with excellent results so I personally have seen little advantage of TTL flash control. I also have the Hassy D-40 which is again dedicated TTL for the 503 but the Metz is more versatile and the D-40 Simpler--The Metz is also much cheaper---check ebay as I don't think it's digital compatible so they now have a Metz Digital version--I gave $150 for the Metz flash LN in the box, all accessories, a Quantum batt. and charger AND the SCA cord/adapter for the Hassy----bargain!!
OTOH if your doing landscapes or other the Mamiya has much better glass--oops!! did I say that !! OK--for me 80 mm vs 80 mm the Mamiya wins!! Handheld the Mamiya wins !! System Wise--the Hassy !! In a studio--the Hassy!