Index Funds & ETFs, if they are tracking the same index, will be the same in an ideal world. The difference would be because of the following factors:
Expense ratio: i.e. the expense the funds charge. This varies and hence it would lead to a difference in performance.
Tracking error: this means that there is a small percentage of error between the actual index composition and the fund composition. This is due to various reasons. Effectively this would result in the difference between values.
Demand / Supply: with ETFs, the fund is traded on stock exchanges like a stock. If the general feeling is that the index is rising, it could lead to an increase in the price of the ETF. Index funds on the other hand would remain the same for the day and are less liquid. This results in a price increase / decrease depending on the market.
The above explains the reason for the difference.
Regarding which one to buy, one would need to consider other factors like:
a) How easy is it to buy ETFs? Do you already hold Demat A/C & access to brokers to help you conduct the transaction or do you need to open an additional account at some cost.
b) Normally funds do not need any account, but are you OK with less liquidity as it would take more time to redeem funds.
