Bit Rate will be determined by the external recording device. You get 10 bit 422 output over the HDMI into whatever recorder (and by extension, whatever codec and it's inherent bitrate) you like. The internal ALL I setting is still very robust at 480/sec but (as with almost all internal camera codecs) it's a very clunky file that is difficult to manipulate in post production with software like Adobe Premier, Final Cut Pro, Davinci, etc...
The primary advantages of using the output are obviously the better bit rate and color depth with C Log, but even moreso the ability to natively record the footage in a very robust ProRes 422 or ProRes HQ that is very easily played back and edited with your typical software. The ProRes files tend to be a bit smaller (particularly 422) than the ALL I. It's NOT for a lack of quality however. The quality is (if executed correctly) better than ALL I even with smaller file sizes. It's simply that the Apple ProRes codecs were designed specifically to maximize capture quality with the most efficient file sizes and compression possible.
When you consider the price of buying FOUR high grade, super fast 256GB SDXC cards vs. buying a Ninja V and a 1TB super fast SSD, it's almost crazy not to record externally, especially when you factor the additional time and headaches of having to either render proxy files to edit, or convert your original video footage into ProRes (like with Adobe Media Encoder) before you actually start editing. You can fight with the original codec in Final Cut or Premier, but I'll never do it again. It's painful.