Devo 7e vs 8s, which one to get?

The 7e equipped with Deviation is probably fine for all 4 channel helis. For 6chhelis, the lack of quick switches may be a limitation for some. We will probably try to address that in the deviation firmware, but the other Devo radios are a lot more capable. Also the 7e has a significantly shorter range. I would not recommend it for anything but minis.
I personally think the Devo10 is the best bang for your buck. It is cheaper than than the Devo8 and has plenty of controls. The screen is easily visible in all environments (the Devo8 screen does not have the best contrast and cna be hard to read in direct sunlight). That said, I still primarily use my Devo8 for all development and testing. That may change as I now have the 12 as well, but I prefer programming the Tx with a touch screen.

But honestly, unlike me, you will likely spend a lot more time flying than playing with the Tx, and you really don't look at the screen much wen flying.

You will not be unhappy with either an 8 or a 10. If you aren't sure about how much you want to invest in the hobby, and aren't going to fly anything other than micors, the 7e will do fine for you.

I need to put this in the FAQ or something considering how often it gets asked

I've now written said article here:
www.deviationtx.com/articles/21-which-devo-tx-should-i-buy

You can certainly fly 6ch helis without issue using a Devo7e. But if you use the 2-position switches for idle-up and throttle hold, you have no access to dual-rates, or gyro adjustments/heading hold. That may or not be an issue for you. We will likely modify the interface to make some of these more easily accessible. I don't know about a 300, but I don't think I'd want to fly a 450 without modifying the 7e range. If you want me to convince you to buy a 7e, I'm not going to do that. I personally think it is fine for what it is, but the bigger Tx are a better value in my opinion.

Note that I spend almost all my time programming, and very little flying. You should definitely take my opinion with a grain of salt.