The M Series is hard to beat, especially for the price. The TCL 8 Series is most likely better, but also a fair amount more expensive, as I recall. I think it's closer to the P Series Vizio in price. The 55" M Series in my living room was under $500 on sale. Really an excellent deal.
These are my couple of differences. The upscaling of the Vizios isn't very good from sources lower than 1080. Fortunately, most upscaling is done before it reaches the TV. If you use an antenna for TV, the TCL has an awesome channel guide with schedule, but you can't directly tune to...
I've bought three 4K TVs in the last 14 months or so. I'm essentially in agreement with my buddies above. TCL 6 series (do NOT get a TCL below the 6 series) or Vizio M Series. They are both well below your budget.
One of the three TVs I got is a TCL 5 series. Like I said, do NOT get a TCL...