Obviously, there are factors which only you can answer, but some things to consider:
- when will you buy "the next" TV after this one? If you're a 2-3 year TV owner, go for the Vizio now and start saving for 75" plus later on. If you plan to have it for 5+ years, maybe hold out for the 75" so that you're not constantly thinking you should have gone bigger.
- besides the size, what do the reviews say about the Vizio vs. the Sony. I have zero personal experience with Vizio TVs, so this would be where I would start
- use a price tracker like
https://camelcamelcamel.com/Sony-XBR75X850D-Ultra-Smart-75-Inch/product/B01A5LU6R0?context=search to see how similar brand / size TVs perform over time (price-wise). While not a perfect 1:1 correlation, you might be surprised at how quickly that $3,500 becomes $2,400 or less.