Im not sure if its an upgrade, but while I was asking around, alot of people say with the 8 bulb fixture, I could keep just about anything. LPS, SPS, softies,
and anemones. The 6 bulb Pro fixture has individual reflectors, though and may penetrate better. Much more light actually in the fixture with the 8 bulb, but not sure if the reflectors make up for this. Ill let you know. Or you could watch my thread. I just asked this question, so you could watch for an answer with me.
https://forums.saltwaterfish.com/t/363762/my-long-term-reef-build-diary
Mark
EDIT- My tank is 21" and would penetrate mine. I dont think the extra 3" will be much of an issue.