The $10 tool will show .75% and 1% chain "stretch" while the mroe expessive tool can show you wear down to .25%. Typically, everyone recommends changing the chain at 1/8 inch stretch over 12 inches, which works out to 1%. I guess you if you want to know how the chain stretch is progressing, buy the more expensive tool. I have the cheaper park tool but if you wanted to go even cheaper yet, a 12 inch ruler would do the job.