I doubt Sideshow would distribute the Medicom versions and make their own at the same time.
Besides, I think the Medicom versions would end up being better and worth the price.
You would see a hike in the price of a normal Sideshow figure like a Stormtrooper, Boba etc because of the armour, might not be as much as the Medicom version, but the product won''t be as good.
Just looking at the Sideshow Premium Fett will give you a hint.
Its nice, but it can't touch the RAH.
That's the other thing, being that SS made a Premium edition of Fett (ROTJ), I doubt they will make a 12 inch version, but again I might be wrong, I don't follow them that closely to know what charactes might get more than one scale.
If they do it would be similar to Premium one and not look as good as Medicom's
RAH ESB Fett shown by Sideshow at Toy Fair 2007:
Pic from www.imatoygeek.com