There are at least 5 factors to consider, to see whether renewing COE for 5 years or 10 years is better for you.

Basically, you gotta calculate what are the costs involved, the condition of your car, whether if there is any consideration to keep the car even longer than 10 years, and all the additional maintenance cost involved as an older car is generally more expensive to maintain.

Here's a comprehensive comparison article thats shows the pros and cons of renewing COE for 5 years vs 10 years: http://www.motorist.sg/article/128/how-long-should-you-renew-your-coe-5-years-vs-10-years