I need a new cog since right now I'm pushing 48/16 and it's really killing my legs on long rides. I'm a college student so I'm kind of on a budget. Anyway, what are your opinions on the following cogs? Also, will I need a new lockring with the new cog?

Avenir Fixed Gear Cog 17 Tooth
Surly Track Cog
All-City 17t
Cobra Track Cog
Formula 17t

I've heard good things about Dura Ace but they seem to be 16t and under so I had to rule them out. Also, if there are any other cogs you guys know about that are good quality and around that price range, please point me to them. Thanks!