Theory 11 seems a bit more likely to accept something from somebody outside the inner circle (not suggesting anything about you) so I would go with them.
Both companies are very cool when dealing with submissions, as both could do a really good job in managing any project you might be interested in doing.
Also consider smaller companies, the Merchant of Magic are great people, Sales@MagincShop.co.uk will get you thru to them, Alakazam and Big Blind are great too.
Before submitting anything, make sure that you're confident that this is something that they will be interested in. With the high volumes of submissions these sites receive you've REALLY gotta stand out.
and how long have you been doing it?
have you performed it live many times?
Make sure you know the ups and downs advantages disadvantages of the trick.
i made an effect i did what Wayne said you should tape yourself near the beggining and a year later tape yourself doin that trick their will be many improvements and changes.
im just getting ready now to submit an effect or so.