Just tried. Both shroomed up, 14 dragon shards for Assassin, 12 for Golem, the price is now about the same, maybe I could put Assassin at 15. But Assassin's DPS is e80, while Golem's is e118. I didn't expect such a big difference, but my first choice was right :)
Now I need 1.1e6 shrooms for the best upgrades and earning 4e4 per reset at lvl 1600 :(
Edit: actually the Wizard is the best when you can buy him his last upgrade. He has a total 2000 multiplier. Phoenix will be the best DPS as soon as I can get him lvl 1000: 11250 multiplier.