Can't say much on the first type as i have no real experience with something like that. In all it really depends on the application... Buut to give you an idea

Aluminium fittings:Most of this won't apply if your only loading the tubes with a small amount of force.

-The aluminium fittings methods is probably more secure however aluminium fittings can weigh quite a bit so depending on how long your tubes are it may mitigate the weight saving of the carbon.

-I'm guessing you would need a lathe to turn the aluminium down to the exact size you want it (+/- 0.1mm at least).

-Depending on your application the bond gap between the fitting and the carbon tube can be crucial. If the joint is structural and will be in tension/compression (technically shear from the adhesives POV) then you're going to need to spend time looking into different adhesives, bond gaps, bond area, surface prep ect).

-You also have to think about how you can practically get your chosen adhesive into this bond gap (sometimes the gap can be very small).

-If using an internal insert - If your tubes are going to be loaded so that they bend in any way then where the aluminium insert is wrap carbon tow around the outside of the tube at 90degrees to the direction of the fibers in the tube if using Uni-directional tube. This will help stop the insert bursting out the tube wall when under bending.

Thought i may as well comment as no one else has. EC have also done a video on repairing a fishing rod which may well be similar to the first option.

Dave.

EDIT: Typo