There may not be a choice
Maybe the $500+ model comes with a lifetime(of the console) Gold sub, the $300 model requiring you to keep subscribed to Gold for as long as you want to be able to use the console at all?
Could work, $200 up front for a lifetime sub may make sense to MS based on typical purchase price and length of subscription for the average user. The cheaper console would allow a lower buy in price but would still net them atleast the same and then even more if the console is kept active for 4+ years.
While they wouldn't make as much from the type of xbox user that has subscribed for 8 years without lapses the increase in the revenues from the typical user may more than make up for it.
Ofcourse this would assume that at $300 the subscription isnt subsidizing the cost of the consoles manufacture and is just a money making move.