One thing to keep in mind on this route is that it is not just full of vacationers. IAH-SEA alone tends to have a lot of elites, and the full IAH-SEA-ANC run is elite heavy because of all of the oil industry employees commuting between Houston and Alaska. Do book the 753 to try and increase your upgrade chances, but keep in mind that depending on the day of the week, there may be a lot of Platinums on those flights. I have been on these flights before when there were Platinums and Golds being turned down for upgrades when they asked for them at the gate before boarding. I've missed it on that IAH-SEA flight before myself.

And, as previously posted, the nonstop 738 is seasonal and ends September 28. After that, the only option to get to ANC on CO is the IAH-SEA-ANC direct flight on a 753, or flying CO to SEA and connecting to AS to finish the trip up to ANC (which will earn you CO miles and 100% EQMs, but does not entitle you to an elite upgrade on the SEA-ANC segment).