I can relate to the comment by mxsailor803 -- keeping reloading costs down.
But, the difference in cost is the up-front cost of dies. This is small compared to other issues.
The notion of having common bullets for two different rifles is also seductive but flawed since you will likely want to use heavier bullets for the .270 Win. You will also use it less frequently since the 6.8 or 6.5 will cover most of your hunting needs.
In that regard, I would look very closely at the Grendel. It will cover your needs from the muzzle out to past ranges where one should responsibly consider taking game. The .270 then can be used with 150 gr or, if you can find them, 160 gr Partitions for the hunts where the Grendel might be a tad on the shy side.
Here is a repeat of an earlier-posted link to an overview of a reloading handbook for the the Grendel: 6.5 Grendel Reloading Handbook
. The handbook offers good advice for managing the gas system of any AR in addition to having an interesting comparison between the Grendel and the .243 Winchester. Oh, and there is load data from several commercial sources too.
The handbook might help you decide. You should have a lot of fun whichever path you choose so it will become more a question what you like.