Renting a Nikon AF-S 300mmF/2.8 VRII versus a 300mmF/2.8 VR1 vs. 300mm F4.0

Ok- so I decided that the 300mm F2.8 was going to be too heavy for me (with the D800) and had decided to buy the 300mmm F4.0.  So I went to the camera store today and both of the guys I was speaking to (one a canon shooter and the other a Nikon D800 shooter) both said if they were me they would stick to using the 70-200 2.8 lens with the latest 2X teleconverter that I already own.  Their feeling is that I might gain about 5% in quality but I would a lot of versatility.  I also noticed that that 300 F4.0 is not on the recommended lens use for the D800.   They both mentioned that it was too bad than Nikon didn't have an equivalent 100-400 lens that Canon has that is a high quality and not heavy.  So I just had a crazy ??thought and wonder what you guys would think of me renting or buying a Canon D70 and a 100-400 lens for my nature photography trips.  Is this a really silly idea or does it make sense? Sorry I know that this is a Nikon form and I love my D800 but I was thinking that perhaps some of you also use or have used a Canon for nature photography

Thanks again,

Marsha