Round Blobs vs Squished Blobs

Two Gaussian generative models assume zero correlation between features. Model A uses covariance $\Sigma = \sigma^2 I$ (one shared variance for all features). Model B uses covariance $\Sigma = \operatorname{diag}(\sigma_1^2, \ldots, \sigma_d^2)$, allowing each feature its own variance. Which model's class-conditional density contours are always perfectly circular (spherical) in feature space?