We knew it was going to be one or the other from the start, so I don't really see the issue. I imagine we'll be able to get Mega Steelix within the game anyway, and if not then it'll probably be available through an event at some point, and then there's always good old fashioned trading.
Also, looks like there will be other ways to obtain a demo code (in North America, at least).
For those wondering, read this for reference:
Edit @ 18:49: More demo distributions are known for the US:
Codes will be distributed at select cinema locations nationwide from Nov. 7–Nov. 13 (while supplies last).
Credit goes to Serebii. Still, it looks awkward to be distributing codes at the cinema of all places, but for any Americans reading you can get a chance there as mentioned.