Dyneema or kevlar for forestay?

That does of course demonstrate the minor issue with just calling it Dyneema- that's a family of fibres, not a single product.

Just so you know... Marlow Ropes are having fibre supply issues with DM20 (MRig) which is why it doesn't seem to be listed anywhere at the moment. Probably all being used in the Americas cup!

If it is running through a turning block and will be static for long periods Vectran can let go without warning. Modern Dyneema products are just as good and will give a bit more warning before they let go. 

I am amazed you snapped it while going downwind, if the mast stayed upright then how was the forestay under any tension at all!?

I'm going top be controversial!
I use marlow sk78 max dyneema on the forestay and capshrouds of my 14. Quarter of the weight of dyform, and similar price once swaging has been taken into account.

DM20 dyneema (M RIG is an english braids product based on this fibre I think) was created for the offshore mooring industry, and has very little creep, but like vectran has lower moduluis of elasticity than dyform or sk78 max (i.e a 3mm rope of a given length will stretch more under a given load)
I use 4mm Sk78 max on the forestay - 3mm would have better modulus and breaking load than 3mm dyform, but I have a 1mm margin for wear and tear. I use 2.5mm on the caps.
Make sure you do good splices, tapered and with plenty of bury.

The one thing I would say is that all woven dyneema products will have an initial mechanical creep associated with the construction and splices. Don't be suprised to take 100mm out of a 6m stay over the first couple of sails. once this is done it will be rock solid. The amount of time that a racing dinghy is tensioned up is negligible in the context of visco-elastic creep.
If you take the mast down and put it up again, the stay will recover some of that mechanical creep again, so will appear shorter on rerigging, but that beds back in quickly.

I'd avoid Kevlar. Snaps with little warning.