Alistair
Have a look at your current client base, the work you are doing now determine what labour cost savings you could make.
Also have a look at what "new" work you could profitable quote for using the system.
As an Old School scaffolder I will always prefer tube & fittings. It was what I was taught from the 1960,s. Yes you can erect ANY job with T & F, but can you make a profit. especially in todays market. The first "system" I ever used was the "Yellow Peril", Oh how we did laugh at this bag of sh*t. It is still been used to this day though. The new systems are well thought out, with designed components that are proven by the manufacturer.
Personally I would make the jump to System, but keeping a stock of T & F and a squad of specialist T & F scaffolders, and charge a premium rate for the "interesting" little jobs that may come your way,
Regards