I think I can clear this one up for you seeing as I buy the stuff for work
There aren't 'grades' as such for any of the main 3 base nuts we use.
I do use different suppliers for the same product based on what it is to be used in and also due to solubility.
One supplier's product might be cheaper but is lumpier but if it does the job in say a product like lawn dressing and lawn granular fertilisers then we buy the cheap lumpier one.
If we are producing soluble plant food then obviously we don't want the powder to have lumps in it or it means that we pay the difference + more in labour, electricity, machine time etc. to crush it up into fine grain, so it works out cheaper to pay for the finer product.
Sulphate of Potash is a different name for Potassium Sulphate. English tend to say Sulphate of Potash whereas Europe tends to be more direct and say the real name (I don't know why we always have to different)
When someone wants K2SO4 they will write Sulphate of Potash because they have always called it that. We will then order Potassium Sulphate from whoever produces the 'grade' we want for the particular product.
Check this link which is one of our suppliers. Thre are 3 'grades' from this supplier and the larger the particles the less it will cost (in general)
http/www.potash-ltd.co.uk/products/fprod.htm
So Tom is correct in saying K2SO4 is K2SO4. It's just different suppliers, different finish, same ingredient but some lumpy some not.
This applies to KNO3 and KH2PO4 also.
When it comes to Chelated Metals this is a different matter.
Although they are all chelated, some are chelated better than others and you do tend to get what you pay for but again, we buy cheaper stuff or expensive stuff dependent on the requirement of the end product.
I just use whichever is nearest. put it in a sample bag. take it home. pommel and granite the amount I need to add to my DI water and then mix. Can't complain for Free!!!
Hope this clears things up
Andy