Hello,
Main difference between Ambit 2 and 3 is that 3 has Bluetooth connection and you can use smartphone as monitor and special HR measurement mode for swimming. This is useful if you actively swim.
Regarding storm prediction, Ambit2 also has it, shows rapid air pressure drop, but not sure storm will come. Battery life no big difference. For my activities (ski touring, mountain biking and hiking) fully sufficient. True, Ambit 2 has two versions. Better one has dual height measurement.
So I'd decide for Ambit2 again, probably get very cheap online.
Regards