Sam, correct, the magazine are interchangeable on both as other parts as well. Other than a few cosmetic differences they're identical. The Firestorm has Glock like sights but the front sights staked, so if you plan to upgrade sights it will be difficult. The thunder has plain 3 dot sights but are dovetailed and much easier to upgrade. The Firestorm has oversized rubber grips and finger grooves wich might help with bigger hands. The thunder grip is straight, so just depends if you like the finger grooves or not. Lastly the Firestorm has a rounded trigger guard and the thunders has more of a point. If you want to ever change sights I'd stick with the thunder. If you don't plan on changing the sights then both are great and basically the same. They both use the same parts and magazines. If you Google bersa thunder vs Firestorm you can view side by side comparisons to see the differences better. Hope this helps.

Well, the guys below have given a nicely outlined detail of the differences between the two so good luck and have a great day!!

Hi Sam, the Firestorm has Finger Groves and Over-Sized Rubber Grips, the Thunder does not. Firestorm Trigger Guard is rounded, the Thunder is scalloped and serrated. Firestorm sights are Glock-like with a dot front post and U outline rear sight, the Thunder is simple 3-dot. Otherwise – it is the same gun. Mags are the same for both. Cheers and stay safe!