There's some 'interesting' comments in the thread.
Parts availability - you can get FAR more for the Z1 than the CB750 SOHC's. To the point you can almost build a replica bike from all the parts.
The bikes are very different.
I would agree the CB750 is a smooth running engine. But very tame compared to the Z1. The Z1 has a very agressive power delivery in stock form, and when tuned, capable of running serious numbers through the 1/4 mile. My old Z1 with 410 cams, 10.5:1 pistons and 4-1 exhaust would run 11.3 at 120mph - and was used as everyday transport. The closest any of the performance street Honda bikes (810cc) came to that was low 12's.
Handling is not great on the Z1 unless you brace the frame and put on serious rear shocks - but with relatively minor frame tweaks the Z1 handles fine up to 130mph.
The Z1 held many speed records in the day - shattered the 24 hour speed record at Daytona in 1973 averaging around 110mph for the 24 hours. Yvon DuHamel ran a lap of 160mph with a 'mildly' modified Z1.
http://www.motorcyclenews.com/MCN/News/newsresults/mcn/2007/May/may15-20/may1607z1daytonavideo/Ease of maintenance - you can remove the Z1 head in the frame - try that with a CB750...
Shims versus tappets. The shims will take a little longer to change - and yes, you need a collection of them - but they work well.
The Z1 crank is incredibly tough. While it is a 'push together' roller crank, once welded the stock crankshaft rods are good for double or triple the stock HP - certainly not something you'd get from stock Honda rods.
Z1 top-ends need to be gone through around 40,000-50,000 miles - the cam chain idler sprockets and cam chain need to be replaced at that point. The crank will keep going for 100,000+ miles It's rare to find Z1's with bad cranks.
They are both very collectable these days.
