Pressure treated will probably last as long, and probably cost a tiny fraction of hard wood. If you want 300x300 in a single piece rather than a few pieces laminated together, you're talking massive money for hardwood. I also think this would be a waste of a scare resource, but that's my opinion.
I got quotes for rough sawn oak a little while back, all 8' lenghts, it was €15 for 6x1, €46 for 6x2, and €115 for 6x3. I can only imagine what 300x300 would cost.
Leo