There's a third, seldom-discussed alternative to opt-in and opt-out: no default.

On first launch or after a set period, show a non-dismissable dialog asking users to choose between the two, with no default option selected, so you can't just click through.

This strikes me as the best compromise between respect for user privacy and acknowledging that data collection can be useful, with the trade-off that some users will choose not to use the software/website rather than decide on an option (thus, 'no default' must be used sparingly).