Combining metaborate and metabisulfite will make same results as using borax and sulfite if you adjust the agents' quantities and the resulting pH. So, it's just a matter of convenience to choose which agents to use.
I've also looked at the description of FX-7 and compared it to FX-4. I see much more significant change in the bromide content and that's more significant than the difference between borax and metaborate. I haven't made up the solution to compare the pH, but I bet FX-7 has higher pH as well. I'm not sure what the point of FX-7 is.