After having all the GB upgrades this is my final set up:
lvl1: magic: all upgrades maxed maxed, with 2 towers
lvl2: shrink ray: all upgrades and all towers
lvl3: splitter: only slicer upgrades, no buzzsaw upgrades, all towers.
Reason for no buzzsaw is that I get more slices and thus more balls and thus more money.
Now I need to wait for more content :P