Ok the question is....
51/20 make up of 67 gear inches and 42/16 make up of 69 gear inches..they have similar gear inches.
I am a newbie. I locate in Sydney I could not find a 42T chainring but I could find a 20T Philwood cog. I am just wondering the pros and cons using big chainring and big cog relatively to small CR and small cogs to achieve similar gear inches? Despite the weight factor.