I am setting here playing with this today. I have my SSS strat plugged into an A/B box then each of the outputs of that going through two identical cables to the Kemper and the FM9. I am using an old Fender American special loaded with custom shop fat 50's pickups in it.
The profile I am using is the Tone Junkie Dirty Surly, the cleanest one in the pack. The amp sim in the FM9 is the Dirty Shirley 2. I turned the gain to 0 and used the master volume for my output.
The Kemper TJ profile uses the Friedman DS cabinet which has a Celestion Creamback 65 in it. The IR I am using in the FM9 is G12H Creamback 421. This is a 4x12 cabinet IR so I expected it to be a little thicker.
At first the Kemper down right embarrassed the FM9 IMHO. The FM9 had such a muddy tone to it. The FM9 just has too much of everything to me and I find myself really cutting everything back to very small numbers to get the tones I like. It took A LOT of adjusting of the amp and cabinet as well as the compressor to get in the same neighborhood as the Kemper. The Kemper still sounds more like the amp. It has that chimey high end to it like the amp does. The FM9 has the high end but it just doesn't have the same dynamic qualities that the Kemper and the amp have. Even after a lot of adjustments it still has this lower mid to it that the amp really doesn't have in this setting. I could probably get it closer by putting an EQ in the chain and dialing it in more. To me, that isn't what this comparison is about. It is about how close the amp sims are to the real thing compared to how the Kemper compares to the real thing.
The difference in the top end made all the difference in the world with how they sound on the in between tones. The position 2 and 4 tones on my strat are much more pronounced with the Kemper. They are there with the FM9 but sound much more stratty with the Kemper.
I would gig either one of the tones I have going right now but much prefer the one I have in the Kemper right now. I didn't adjust the amp at all from the original TJ profile. I just cut the effects way back because he uses much heavier effects than what I do. I am getting tones I like from the FM9. I just have to dial EVERYTHING way back in it to get there. It takes a lot of adjusting and testing to get it where I want it to be.
My honest opinion is that the amps sims in the FM9 are not all that close to the amps, at least the ones I have tested that I have the real amp setting here to compare to, I have a few of them. This isn't a huge deal though. I have been able to adjust them into something I can get along with. I would ignore the name of the amp they are modeling and use your ears to find one that is close to what you want and adjust from there.
I think the FM9 may satisfy a higher gain metal player more than the clean / low / medium gain players. I think the Kemper will be the unit for the guys that don't like the endless tweaking and prefer something that has a signal chain closer to using a tube amp and pedals. To me, the Kemper does a better job at providing the actual tone and dynamics that the real amps give you.
The bottom line for me is that I will keep both of these units because I am able to do that. They each offer me something slightly different that I can use. I will factor in that I have to make a lot more adjustments in the FM9 to get the tones I am looking for. I don't mind doing that. If I am doing a gig where I need my piezo equipped guitar, I will take the FM9. If I do not need the Piezo, I will take the Kemper.
I am glad you asked this question. It gave me another chance to try getting another specific tone from both units and comparing it to my amp. That is always a fun day.