This has been discussed at length in other threads, but I'll try to give you a fair reply with my 2 cents...
First, it sounds like we agree that DIN is the way to go.
1. I prefer LP tanks because I can always get a good fill.
2. When doing partial pressure blending lower bank pressures are required, so it's more convenient (ie: it will be easier to get nitrox fills).
3. It is not uncommon for people to intentionally overfill LP tanks. While it would be unprudent of me to
suggest that you do this, I can say that it has been often enough to strongly suggest that there is very little statistical risk when filling a
good condition LP tank to say 3000 psi.
4. I personally would run for it if anyone tried to fill an LP tank to say ... 4000 psi in my presence. There are others who are comfortable with that arrangement.
In any case, I wouldn't panic about the possibility of people occasionally overfilling your LP tank.
Some would even say this is a good reason to buy an LP tank in the first place
You might reconsider buying 120cf tanks however. I would personally choose an LP 104 or 112. Both LP and HP 120's are a little oversized.
Best of luck with your decision whichever way you go.