The King cab is the XE. The crew cab has the 4x4 and the towing package.
Actually, you can get a Titan XE king cab in 4X4. It's been like that since the Titan debuted.
To the OP,
the king cab has a few pros and cons.
Pros: it's more proportional looking, has a longer bed, no B-pillar in the way when all the doors are opened, smaller cab space which feels more intimate and sportier. Cons: squeaks and rattles due to no B-pillar support structure, legroom is more cramped in the back, rear door windows don't roll down.
The crew cab has a few pros and cons:
Pros: full sized legroom for backseat passengers, stronger cab structure meaning less squeaks and rattles due to B-pillar support. Cons: looks unproportional in some angles, negative "family man" image due to 4dr configuration, short bed is useless if you need the length of a fullsize truck bed.
I got the king cab because I didn't want the family man image that the 4 dr configuration would evoke. Since only two people at the most (including me) would ever be riding in my truck, I didn't need a 4 door crew cab. Since I need more space than a single cab, but not quite as much as the crew cab, I went with the KC because it fit my needs perfectly.
Actually, the KC's are cheaper than the CC's when comparably equipped. On average, it's about $2k you'll save in purchasing a king cab compared to a crew cab. Also, the crew cab weighs about 250+lbs heavier than a indentically equipped KC.