Well call me insane but I've had all three. LOL.
I've had the F-75 longer than any machine I've ever had and that speaks volumes for those that know me! It will take a hefty machine to make me lay down the F-75 for it. But you were asking about the F-70 and T2 so I will start on those.
The T2 I had was one of the first generation ones. I really hope they have fixed the problems they had...as I know they have with the F-75. The problem I had with my T2 was the battery compartment deal. If you bumped that thing ever so slightly, the machine would reset itself. I mean you could do it laying the machine down to dig and as we all know how prone they are to falling over on the side...just that little jolt would reset it and you'd have to go back through all your settings again. So if they've fixed THAT particular problem with them, I would give the go on the T2 over the F-70. It is very similar to the F-75 and I actually like the more open iron range on it than the F-75s short iron ID range. It seemed to do better in the iron dirt ground (that reads above the .3 on the FE meter) than the F-75 does.
Now going to the F-70. At first I kinda liked the little machine. In air tests and playing around in the yard, it was an alright little dude. In the real world, I tried hunting with it myself but always ended up taking it back to the truck and getting the 75 out. I hunt in hot ground and the stock coil on the F-70 is junk for hot ground. So naturally I put the F-75 coil on it and to be honest, it does help it drastically! But I found even with it, that it wasn't as depth capable as the F-75 and it did false quite a bit in the hot ground even with the F-75 coil on it. The meter screen was flimsy, I personally don't like the grip on it, and overall I just don't think it is worth $650.