SR2 is a little bit more serious than KSP, I'd say, but it's no Orbiter either. They use a smaller-size solar system (bigger than KSP though) by default (you can swap it out for a community created realistic system in-game), so you can still brute-force to orbit or build a SSTO and it will kinda work.
What I love about SR2 is the aerodynamics and the procedural parts. You can create an airplane and design its wings and control surfaces by hand, you can play with the center of gravity, custom fit it with a jet engine (by tweaking the compression and bypass ratios, adding or omitting an afterburner) and just fly some aerobatics.
You can use electric powered rotators and hinges, powered procedural wheels for cars and other cool stuff. And if everything else fails, your rocket is saved is a plain and simple XML file, so it's easy to just dig in and tweak some more.
On the other hand, there is no campaign yet and only a bunch of tutorial-like-tasks to get you started (take off, reach speed X, achieve orbit, hit the Moon with your spaceship etc).
I wouldn't say that either game is better, SR2 is just different and provides you with some more freedom and options than vanilla KSP.
Also KSP is Windows + Mac + Linux + consoles, while SR2 is Windows + Mac + iOS + Android, and the mobile versions are surprisingly playable (especially on tablets or larger phones).
