Both loans have their own advantages and disadvantages. Your intention is buy a car at cheapest price possible. Hence, we tend to look for low interest rates. The interest rate on a car loan generally ranges between 8.60% to 16% depending on the credit profile of the customers. For a fresh car loan, you may have go through the documentation process and it could take some time for approval after submitting the application. 

On the other hand, a top-up loan on existing home loan is like getting additional amount on the current loan. The interest rate ranges between 8.40% to 12% which is slightly lower than the car loans. However, getting a top-up loan depends on your past payment records. Continuous delinquencies may not be eligible for a top-up. There are chances the top-up loan interest rates could be slightly higher.

Having mixed credit products can increase your credit score as it is considered as one of the parameters in credit score calculation. Compare both the options well and choose the option that suits you the best.