Sales and Trading by Categories: Fixed Income, Commodities, Currencies, and Equities

I understand that there might be a need to develop a specialty when it comes to being an analyst/associate with S&T. I just had a few questions as I am in college right now and would like to get a more relevant handle on the topic. In terms of the four categories:

Which field has the greatest demand for more analysts?

Which field is the easiest to jump into entry level after two prior IB internships?

Is there a significant difference in pay/lifestyle in one versus the others?

Is there any credential (other than experience) that could set me apart from the rest of the competition to gain entry into a field (a license or particular achievement)?

I really appreciate the help. Thanks a million guys...