You would be well advised to take the upgrade. As has been pointed out command experience is definitely at a premium. If you join EK now you may love it. But if you don't where do you go then? If you have reasonable command experience it gives you many more options if you decide the sandpit is not for you.
Also as has been posted elsewhere, the upgrade in EK is not automatic. I would argue that your chance of success is much greater if you already have command experience. Short haul builds that much quicker with multiple sector days. I would be amazed if EK tell you that you can never reapply because you turned them down for a command. However either way you are a lucky guy.