This reads like another promotional price, so keep an eye on your statement(s) for another "X Month Service Agreement."
What Cox wants is for you to NOT pay a lesser bill, so they invent plans to keep your costs the same. 250/250 is not a plan. 250/250 is invented just as Cox did to me from 150/10 to 200/10...to keep me paying the same price.
Did you ask Cox about their 150 plan? It's on their webpage for Speeds and Data Plans, but where is it in their offerings within your account?
You need to keep an eye on Cox.