Firefox Panorama has a unique way of organizing tabs into groups so that you can work with a loot of tabs (really lot of them). While the Side Tabs experiment is totally disjunct concept, The effective use of screen real estate is key measure in both. With just screen real estate, its still Panorama that wins the battle.
Side Tabs seems turned out to be a sore experiment. For one thing, it may be designed only for netbooks. There are other considerations for developers perspective. Many a time, the height is considered variable. The width, however is counted to be under standard 800, 1024, 1600 etc, those of the video resolutions. Many fixed layouts are drafted to work on these width numbers. Free flows however do not have this issue. Now, having Side tabs in picture, it creates two sets of fixed layouts to work with.
Firefox Panorama not only tackles width issue in question, also provides a organising benefit. So Which one is it? Most probably Chrome will eventually mimic Panorama.