Okay, I'm torn. Which of these games should I go for? Star Wars or ....planet wars?

Wow, they're both great but so different that it's not an either/or decision - you should get both. If you really can't get both right now I'd say go for Star Wars first and by the time you've played it through maybe you'll be able to get Universal Sandbox, which has endless playability.

I would go for starwars until i could afford Universe Sandbox ²

Don't bother with Universe Sandbox -- get Universe Sandbox 2.
As far as what to get now ... no idea. Flip a coin.

I Have played both, I loved Universe Sandbox, how ever there is a new one called universe sandbox 2. BUT that game is only really fun for about a max of 20 hours spread out. I would recommend Star wars Jedi Academy, It is by far my favorite of the star wars games, really close to star wars battlefront 2. It has a lot of story to it and the multiplayer was really fun. you could customize what powers you want and make them as strong as you want while playing with who ever you want.

Jedi Academy is the best lightsaber simulator until now (you can use blasters too). It's a nice game for it's time, with ok graphics, nice single player campaign and multiplayer. Universe Sandbox isn't that fun.