A: They're basically the same.
First of all, you need to add all this stuff to the RED SCARLET-X to be ready to shoot:
Second of all, the list price of the Canon EOS C300 is $20,000. Once it hits retailers like B&H Photo, that will come down. Let's take its closest competitor, the Sony F3, as an example: the camera's list price is $16,800, but B&H is selling it for $13,960 ($13,160 after a rebate). So let's take 20% as the estimated street discount, which brings the $20k camera down to $16k (which has also been a reported selling price of the C300 -- probably due to confusion between MSRP and street).
The RED SCARLET-X lists for $9,750 and that is direct from RED: no discounts. However, that is a "brain only" price (though it comes with a SSD recording module, Canon lens mount, and AC adapter). After December 31st, in fact, the RED's prices will go up. But if you include the prices of the things you'll need to shoot in each cameras' most basic configuration, many all of these things are included with the Canon but not with the RED. (A couple of them are not included with the Canon either, where noted).
- Camera Body
- Lens Mount
- Side Handle (RED +$950)
- Viewfinder/Screen (RED's 5" LCD is $1,600, though it's probably nicer than the Canon's EVF)
- Media Module
- Media for 1 hour of footage (a cheap $100 CF card for the Canon, but two $950 SSDs for the RED)
- Media Reader (a cheap $50 CF reader for the Canon, but a $250 REDMAG for the RED)
- Battery Charger (an extra $150 for the RED, though I'm not clear on whether the included AC adapter can also charge batteries)
- Batteries for a few hours of shooting (an extra $150 battery for the Canon, three $195 REDVOLTS for the RED)
This brings the RED up to an even $15k. As you get into more realistic battery and media numbers -- I included the absolute minimum above -- the SCARLET-X becomes more expensive than the C300. Which is to say nothing of the fact that the C300's 1080P MPEG-2 files require much less computer power to edit and process in post than do the 4K R3D files (thus more expense on the back-end for the RED). Plus, I'm betting the Canon will be better in low light situations, which makes your necessary lighting budget lower (though just because you can shoot without lights does not mean you should).
Finally, while some pointed out during yesterday's brouhaha that "the RED will be available sooner," that remains to be seen -- RED's November 17th (for the PL mount version) and December 1st (for the Canon mount) dates are for when the SCARLET-X starts shipping. RED's history is not one of shipping on time (not in any real quantity, at least), and on top of that some of the required accessories are backordered, so by the time you're able to get your hands on a full SCARLET-X kit, I'm betting the C300 will be shipping in volume.
Certainly by the time RED has all of the SCARLET-X features enabled -- including a basic ability like playing back the footage in-camera (yes, this is not currently possible with the $28,000 EPIC) -- you can bet the C300 will be out in the wild. I'm not second-guessing my decision to order a SCARLET-X, but let's not pretend that the RED will be ready to shoot for $10k on December 1st. For all intents and purposes, both of these cameras are $16k (without lenses), and both of them are shipping in 2012.
But only one of them's being used to shoot The Hobbit in 3D!
UPDATE: some of the comments here and elsewhere have caused me to write Why I'm Ordering a RED SCARLET-X, and How it Relates to My Feature 'Man-child.' Hopefully that clarifies things.