Have you seen this article?
Best picture quality 70-75 inch TVs - Summer 2017: Reviews
You should aim to get one of the recommended TVs instead at 75" if you can, not the P series. Not that the P series is bad, just that it seems average.
So the differences? With the E7 you get much better sound as it has a Dolby Atmos sound bar included. If you don't need the sound you could look at the B7 or C7 instead. You get deeper blacks, much wider viewing angles, Dolby Vision HDR support and hardware that supports HDR up to UHD premium standards.
With the Vizio you get more options to smooth out motion, although both perform the same out of the box and a larger screen.
Optimally you want to view as close as 5 ft to a 75" TV and 4 ft to a 65" if you want to take full advantage of noticing the increase of UHD resolution vs FHD.
They say you should always go for the bigger size, but I would base your decision on viewing distance and also whether you can afford a better 75" tv instead such as those listed in the link.
I would also beware about prices, you say you need to buy today but you will likely save a lot by buying later. Right now prices are high and there will already be big discounts come black friday. Between black friday and spring next year, those tv's will be at their cheapest potentially saving you big $$$s. You will also likely be able to afford better TVs then compared to now as there is a chance they will drop into your price bracket.
If there were still some discounted 2016 models about, I would recommend going for those instead to get better bang for buck, but likely they are all sold out by now.
Hope this helps.
