ISP advice needed, choices are AT&T and Comcast/Xfinity. [View all]

Problem is AT&T max is 50 Mbps down and I want to seamlessly stream 4k HDR on two separate devices at the same time, routinely.

I have Suddenlink now with 130 Mbps down and never have issues doing this.

Xfinity offers up to 400 I think, I would get 250 probably.

3 bed 3 bath, one story, 2000 sq ft. I currently use an extender with a Nighthawk Netgear 2/4/5.0, not sure the model. $200 at Costco 3 years or so ago. The extender is I dont know what, not home.

What should I do and if I use Xfinity and tell them I want an extender?

Why do the Xfinity internet ads say UP TO, I know what that usually means. UP TO 250 Mbps...?