which bike: KHS Flite 100 or Grand Royale Lurker

Just letting you all know i bought the Flite, it’s 55cm and peppermint colour, a bit girly looking but for the price i’m a huge fan. I’ve taken it out for a few rides already. I’m loving riding fixed. I did shit myself when trying to hold myself from flying down a hill and have the rear cog slip on me! i’ll grab a lock ring tool tomorrow and fix it up. I’ll also get some brakes so I don’t die learning.

some people mention replacing the rear cog? why is that? and any other recommendations on what i should upgrade?