Upgrades for Ground Main Defence unit

I want to upgrade my Ground Main Defence units, and the question is - which upgrade to buy?
Since those GMD (Ground Main Defence) upgrades are expensive, i cant afford all of them. I have almost 24k SPs, i have option to buy LUCKY GMD. Should i collect more SPs and but FASTER GMD?
Which one can make a noticeable difference in gameplay?

Ofcourse.

Buy Faster GMD, it's the main upgrade of the game.
Cheaper GMD is really good too but buy Faster GMD first.

Hey tnx for answer!
I guess i will be able to buy it when i reach rank 7...25k SPs now, still dont have an option to buy it.