The difference between airlines is smaller than it use to be. I would stay on United and use the benefits of 1K. I had the same decision for many years as I was flying 250-300K PQM a year and could have easily gotten to 1K and Delta Diamond every year. However, by flying United I got to use the benefits of 1K. Upgrades are slim but 1K has other benefits on award tickets that are worth it.