The mods in it and the fact that its newer.
That sounds like a snarky reply and its not really intended to be, just a reminder that the question itself is a bit odd to build an answer for. All modpacks are different from one another if they have different mods and configs. And if it falls into the Kitchen Sink category (?) then its almost by definition limited to those factors in terms of differences.
The more you get away from those factors, the less it becomes a KS pack and the more it becomes a themed or niche pack.
Well, yes. Your answer is snarky and my question is stupid if we read it without the rest of what I explained.
Also I normaly have problems to reduce my explanation to the important stuff and people get distrected by so much bla bla from me
So lets try again.
Since a GregTech Mod is in the Pack, I don´t think that this is for everyone. But since FTB Flagship Packs are usually kitchen sink packs, this does not fit. (as far as I can object)
I started SkyFactory 3 just yesterday. But I know I will fall for the hype of FTB Beyond and will play it as well. So if 90% of the Mods are the same in both packs, I would like to know this now so I don´t have to start over again in some weeks.
Note that this is not a problem of any Modpack or their content. I´m nut judging any of this and there is no Problem in having different packs with similar content.
Its just for me personal decision making on what to spend my time the next weeks
Aaaand I can understand that no one will give me this answer as well
But I gotta try.