Apple charges much more for the same RAM then others do. Buying from an alternate shop is far more cheaper. However not every RAM will work: search for RAM which is tested to work with Apple machines.
Crucial have a system checker on their site and also offer a guarantee that their ram will work, seems a no brainer really, for £70 per 8gb I may even max the machine out.
You can't go wrong with Crucial. Excellent quality and price. Remember, buying the cheapest is not necessarily a good idea, particularly where RAM is concerned.
As long as it's from a known brand and reseller and comes with lifetime warranty, it should not matter. RAM failures are pretty rare and there isn't really a way to say that this RAM is higher quality than that (haven't found any RAM durability figures). Personally, I always go for the cheapest brand if I have heard of the brand before (I would say I know all bigger brands).
No doubt that Crucial is great but if there is a G.Skill kit for less for instance, don't be afraid to buy it 😉