How to test for Overall Significance?
Assuming that the "best" model is the model that makes the most accurate out-of-training sample predictions, there are two types of measures that you can use. R squared will only increase as you add more variables hence it is not a suitable measure.

There are two common approaches:

(1) Indirectly estimate test error by making an adjustment to the training error (e.g. Adjusted Rsquared / BIC/ AIC). In your case, the simple linear regression performed slightly better in terms of Adj Rsquared.

(2) Directly estimate the test error using either a validation set or cross-validation.