Title says it all which game should I get and why.

Both games look amazing Flower looks relaxing

while Pixel Junk shooter looks artistic and plays well

I'm getting one of these games along with Final Fantasy 9

Pixeljunk Shooter is longer, Flower is overall better as long as you "get it" but if you don't you'll probably hate it and think it's stupid (see Hatmoza)

Here's a good qualifier:  If you just want some really fun gameplay and like seeing physics interace than Shooter is the way to go, but if you're someone who appreciates storyline and a sense of atmosphere and art then Flower is simply beautiful.

Flower is something everyone should experience. PixelJunk Shooter is a fun game with more content than Flower, but I felt it was somewhat repetitive and got old pretty fast.

I'd go with Flower.