There's nothing wrong with a Tin roof & roofing couplers Phil so long as you can provide a safe means of access for fitting the tin sheets and purlins..
I'm personally not a fan of shrink wrap on roofs simply because people do not fix the patch ties in the correct places, the sheet just gets rolled out from one side to the other and fixed on a ledger or handrail at each eave.. you get a massive concentration of load at these points which you just can prove. You need to be fixing patch ties at 1.0m centres along the beams/purlins.. and the suppliers never tell you that when they are selling it to you!!! Same applies to independents as well... (shrink wrap is a subject for another time..)
As for the system, well.. thats a tough one. Suppose it depends on what stock u have already and what you are used to or like. Personally.. I don't think there is a better product than Layher at the moment. Yes its expensive, but its a quality product, they provide good support in the UK, have lots of additional components and the rest of the product range is better than all the other systems in my opinion.
Thats not saying that UBIX aren't good as well.. but I don't feel they have the same all round package as Layher, since Tony Jenkins left ASP I don't think its been the same. But if someone knows different I'm happy to stand corrected.
HAKI... haki... haki.. I will never understand the reasoning why they made a 750 beam with a double chord. They are very popular, and a few years ago I may have said they were the market leader, but at the moment I don't think they are. They don't have the technical support of Layher or UBIX for starters.
We design temp roofs with all those products, they all have their strengths but if I were buying with my own money.. I'd buy Layher. That may change in the future, but right now, thats my opinion..... and like I said, I'm open minded.
If you ever want any advice, you can always call us, we'll help if we can.