as many lvl6 as possible to start, then upgrade later if need be. I went for volume rather than gem level and it was fine tbh. Just finished all levels shining :D
I really did get bored of this game quickly....really annoying chasing down a ship. Move stop fire move stop fire move stop fire.
Could be good, but still needs a *LOT* of work. Didn't bother to get the Hard badge as it was just too much effort waiting for another 10waves or so.....as soon as I was past lvl20 I just stopped building.