Neither is stronger, but each has it's advantages over the other depending on what application it's intended for.
Carbide could be used to drill something that has already gone through heat treat, or it could be used in an nc mill, it will drill a very accurate diameter that often will not need to be reamed afterward. That is to say if wanted to make 50/ 8mm dowel holes you could use an 8mm carbide drill without having to go back to ream. Careful though carbide is very brittle and easy to break, also once a carbide dill is worn out you might as well toss it, because the cost of a re grind is more than the replacement cost.
Cobalt is ideally used in regular drill press operations, and will hold sharpness much longer than say a high speed steel drill bit.