Here's what SS has to say about that:
Q: I just saw Medicom’s new RAH R2-D2 and C-3PO. They’re awesome, but I want to know if I should order them or wait to see the Sideshow versions? Are you going to make the droids in 1:6 scale?
A: Medicom’s Artoo and Threepio are very, very cool! We’re excited about them too!
We have great respect and admiration for Medicom’s RAH Star Wars line. They bring their own unique style and flair to every product, and we believe that Sideshow and Medicom sixth scale Star Wars products are wonderfully complementary. The two lines do have a slight scale difference (Sideshow standard male bodies being slightly taller than Medicom), but we have witnessed wonderful collections that include figures from both lines.
Sideshow and Medicom do share information about our line planning, in part to avoid releasing the same characters at the same time. This also helps us to differentiate our products - we do look for ways to provide something unique in our products, as does Medicom. For example, Medicom has released their Ep 3 and Ep 6 Darth Vaders, contributing greatly to Sideshow’s choice to focus on the Ep 4 version of Darth Vader. While Sideshow is currently in development on the droids for 1:4 scale, they are not currently in development as 1:6 scale - in great part because we believe strongly in the quality of the Medicom figures. It is possible Sideshow will develop the droids in the future, but they do not appear in our 2009 or 2010 planning.
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If you want the droids in your 12" collection, I would get these now or be prepared to wait several years for a preorder, then another year or so before they start shipping.