Membership rates are usually a free sign-up. Yes, sign-up and get a discounted rate. Don't forget to unsubscribe to their endless marketing emails though.

Keio plaza and Cerulean are both good hotels. Both are aging I think. Keio plaza only had some rooms refurbished recently and Cerulean had its executive floors refurbished recently.
In terms of location from Keio plaza it's about a 10 minute walk to Shinjuku station, before you spend another 10-30 minutes in the maze of the station. Cerulean is right next to the station, Shibuya station is almost as big though. From both Shinjuku and Shibuya you will need to take two trains to get to Disney though. Lots to do at night in both areas, probably more happening and more eats around Shinjuku I think; the crowd around Shibuya are pretty young.