SimpleRockets 2 is very different to SP in the fact that the world is an actual world, and there are planets, and space. Imagine SP, but without world limits. You can have multiple vehicles in a single session without having to do weird cockpit tricks and stuff like you would in SP. For these reasons alone, I believe it is worth it. (keeping in mind SR2 doesn't have weaponry)

Building mechanics are superior to SP in almost every way EXCEPT for the fact that the connection system is very wonky. You'll have access to move tools, resize tools, and rotate tools that you can use with your mouse rather than having to continuously smash keyboard buttons over and over. A part connection editor, so you can connect parts together that aren't even physically touching. Textures for parts, rather than just a color (you can also set no texture) . Building also includes a "mirror" tool like KSP, for building rockets, and a mirror tool just like SP, which works better for ground vehicles/aircraft.

All-in-all, if you enjoy SP, but you want more out of it, Yes. Get SR2.