Why is Slughorn raving about Lily instead of Snape?

[Florentine Maier]

No: HPFGUIDX 133951

I find it highly peculiar that Slughorn constantly compares Harry's 
potions ability to Lily's. Slughorn must have been teaching Snape too, 
who has been in the same year as Lily. Wouldn't it make more sense if 
Harry reminded him of Snape?

Possible theories...
a) Snape invented the improved potions recipies but never put them into 
practice in class but passed them on to Lily instead. (Highly, 
unlikely, since Snape apparently didn't like "mudblood" Lily.)
b) Lily found out all the potion improvements and practiced them in 
class. Snape copied down everything she did in his book. While he had 
been an average student, he later became a great potions master by 
imitating Lily.

I'd love to know your opinion.
Florentine