Even though I'm IAH based the Delta option is definitely not for me. Their miles are worth much less than on UA.

Best premium cabin can vary quite a bit after you factor in international aircraft regularly getting thrown in on some routes. Also, a lot of people say the lack of UA's wifi is a big deal. The lounges here in the US are so underwhelming compared to the rest of the world that it probably doesn't matter.

Inter-US flights do have free upgrades available. There are a lot of flights to other countries that offer that as well, just to central america or Canada.