I bought the KSGER 3.1 about a year ago, but I have been reading that KSGER's have been having issues, as of a couple of months ago. There have been some complaints about runaway temperatures where the tips overshoot the temperature and will actually glow red. Mine has had no such problems. I'm not sure what's been changed since I purchased mine, if anything.
I am using the stock FW and it works well for me. The pre-set calibrations for the T12 clone tips that I have are right on the money as far as temp shown vs. actual temp at the tip (I have an FG-100 clone to test tip temperatures), although I have read that some people have had issues with real Hakko tips and others have not, so if you plan to use real Hakko's you may want to consider the Quicko and the CFW. I'd say if you are only going to use clone T12 tips, there's no real reason to use a custom FW, IMHO, unless the one that comes with the newer KSGER's is as buggy as I have seen reported lately. If you would like to have the flexibility to use other tips, like JBC, then I would have to agree that you should look at the Quicko suggested in the CFW thread by DavidAlfa because the CFW that he developed works best with those stations. Yes, it does work with the KSGER's but as I follow that thread, there are some anomalies that are peculiar to the KSGER's and since DA bought the Quicko and does not have the KSGER, he cannot directly troubleshoot the issues, and since you have not bought the station, yet, why not just go with the one that works best with the CFW if you plan on using it?
One thing to keep in mind is selection of handle. I have the SS one and it works well enough, my main complaint is that there's no indication to know you pushed the tip in far enough, its real easy to push the guts out the back end because you pushed too far. If they could make it so there's a definite feel you're seated or an audible click, it'd be a good handle, IMHO. There's discussions on handles in that CFW thread as well. If you plan to use clone tips, keep in mind that they take a few minutes to "break in" before the station gets an accurate tip temperature reading. There have been many discussions on that. Once broken in, as I said earlier, the pre-set calibrations work well with the clone tips, at least those few that came with the unit I bought.