For me, an onion is an onion, is an onion, is a .........
I buy mine usually from Somerfeld, where they have been under 30p a pound for years now.
I've tried Spanish, blue, big, small......, but never noticed any difference at all in the curry.
For those who have read Bruce's Curry House article saying that restaurants use only Spanish onions, - well where he got that info from bewilders me.