Having just purchased copilot and a user of Tabnine in the past (using Phpstorm) here is my own personal opinion. The suggestions offered by Tabnine weren't anywhere good as copilot - not even by a long shot (at least for PHP and Javascript).

But almost an year has passed since I used this so I don't know if they have had a major update and the results are improved, but previously 1 out of 10 tabnine suggestions were barely useful, compared to 9 out of 10 with copilot.

Since it's a paid offering now I really hope they've upped their game.