I can answer your question. In the last few months I switched to commando style chinups, for two reasons 1) I think it aproximates rope climbing better than most other types and 2) the bar I ahve spins around so it is always rolling away from me with regular chins but with comando grip my hands opose each other and hold it in place. Now, since I started doing only commandos, I ahve noticed my biceps have grown more than any other time doing pullups or chins before. I think it is the peculiar position of arm closest to the bar that seems to isolate the bicep more when done this way. My traps also benefited a bit mroe I think, though not as much as the biceps. I had some days in the beginning when I over did it and had sore biceps, and I am already a chinups vetenren and cannot remember the last time I accidnetally got sore biceps from chinups, even with heavy weights.
How to do the commandos. Stand under the bar and just to one side so that it runs front to back. It should be directly over on of your shoulders. Reach up and grab it with that arm, your palm facing towards your body. Reach across your body and grip the bar with your other hand so your palms face each other. Next hang under that one shoulder and pull, keeping one arm directly under the bar while the other arm goes across your body and grips it it that way. Near the top of the movement, the arm going across the body seems to be pushing a bit rather than just pulling, due to the angle of leverage. Since this works the two sides fot he body unevenly, I always do two sets together for each about half as many reps as I would normally do. So for example, I wil stand witht he bar over my right shoudler and do a set of 8, wait 20 seconds, stand with it over my left shoulder and do a set of 8, and so on.
I have been doing:
8, 8 (20 seconds in between each 8)
(wait 1 minute)
7, 7
(wait 1 minute)
6, 6
(wait 1 minute)
5, 5
(wait 1 minute)
4, 4
(wait 1 minute)
3, 3
and just recently added my 10 pound weight (the only weight I have).
So for a better bicept workout, throw some commando grips in.