Looks like the 150mm it will be then. Everyone seems to be in favour.

Excuse my stupidty (still learning the fineness of MF) but what are extension tubes for? A few people have saide I'd need one with this lens but I'm not sure why? Are they like "x2 Converters" in 35mm, making a 50mm to 100mm but with typically one or two stops lost? Again, I looked at ffordes.co.uk and found this, which is a 'Hasselblad Extension Tube 55' :

http://www.ffordes.co.uk/product/12112717332081

No specific mention of "8 or 16" extension tubes? And I'm not sure what sort I'd need for 501CM body?