A trendline is most accurate when its R-squared value (R-squared value: A number from 0 to 1 that reveals how closely the estimated values for the trendline correspond to your actual data. A trendline is most reliable when its R-squared value is at or near 1. Also known as the coefficient of determination.) is at or near 1. When you fit a trendline to your data, Excel automatically calculates its R-squared value.

So, second formula better fit your data.