Dave

If using 1x19 I would use 3mm sidestays and 2.5mm forestays. The Japanese wire is best - obviously 316. It's hard to get these days though. If you're still buying in imperial it's 1/8 and 3/32.
2.5mm diform can be used on both side and forestay. It's as strong as the 3mm 1x19 but has less windage. Personally I use dieform sidestays and 1x19 forestays and replace them annually or about every 50 to 60 races. I do my own swaging so cost is no great factor. The only issue with swaging diform if you don't have the equipment to do terminal swages is thimble swaging is a bit tricky and time consuming. It's OK with practice and a template.

Cheers
John Dowling
AUS9