The Offset is MUCH bigger...I like the offset's lock more, but I prefer the manual and spyderhole of the captain. In a couple months, Offsets will be available in G10, and I think the molding of Kershaw's G10 is great (as well as the texture of Spyderco's). There is a big price diff in favor of the Spyderco.
I don't find them similar myself, but if you want one, I think the big deciding factors will be price and size. The Offset is massive and much more expensive. If you want Kershaw's interpretation of the Manix, the Offset might be where to go, but for an EDC, the captain is the clear choice. Both very cool...the captain, I think, is underrated. Hopefully I'll get to handle on in real life soon.