Get the fastest, cause you might not always be stuck on some undersped internet connection. If you get the low end, then next year Xfinity in your area stops sucking, or you switch to a better provider and suddenly you could be getting 900mbs except you got the slow one. Especially since it’s a USB you could be plugging into any computer for years to come.
900mbs internet must be good for when you have 5 people in the house all streaming movies.