"What can you tell me about the different possible tracks a person would take at the Big 4 in regards to tax vs. audit? Is there a difference in career progression among the two specialties? How about career potential? Workload?"
Loaded question. In terms of career potential, they're both pretty potent. Audit leads you to controller/accounting manager/ VP - accounting positions at companies if you choose to quit. Tax can lead to tax manager positions at various companies/firms..to help them minimize taxes and exploit tax loopholes. It's a pretty important position in many companies...I'm not an expert on tax careers though, so you might want to ask somebody in a tax related position.
In terms of career progression, it's the same as audit...start off as a staff associate...move to senior..manager..snr mgr..partner.
In terms of workload, my friends in the tax department seem to work two busy seasons...in the fall and in the spring. So I think they work more than in audit, but they are more flexible in that the work can be done from home and there's very little, if any travelling.
Hope this helps, sorry I couldn't provide more information about a career in tax.