I am still currently using Logic 9. I am sure that I will get Logic X within the next year. This is my reason for keeping Logic 9. Control. All of the software providers have moved to subscription base and software DL. This is good for them, but annoys me. I like to know that I can disconnect from the internet and be able to load an use the software. I know this will only be good for a short time. Because of the activation process. But I will keep mine until it won't work on this computer and can't be activated in the future. If it gets to annoying. I have a computer / anolog combinaton that I will revert back to. There is something about physically having the discs and loading them that gives a since of ownership. DL from site makes it clear that you are just renting until the new version comes along. They don't need to tax the internet. It's built into the system.
That's my rant for the year
If you are happy with Logic X, and don't use Logic 9 anymore, and/or if you need the money, sell. If you still use Logic 9 for some sessions, keep. It sounds like you are not dealing with clients, but only using Logic for your personal music/work. Assuming this is the case, the choice is completely yours.