Regional has great offerings as does the B4, I chose Regional Top 10 over B4 given they paid more at the time and I liked the people better.  Chances are (B4 or not) you will end up leaving PA within 6 or so years so the best thing to do is get the large name on your resume to get your foot into the door in private industry.  I had no trouble securing multiple offers and positions prior to leaving my old firm, but then again that was a different time as compared to the current.  Generally speaking most people that I personally know that work in PA (for the most part) actually enjoyed working for a small firm as compared to Regional and B4, so its really a trade off.

Point is you can either give up a little now to possibly gain a little in the future as far as lifestyle, the Company where I work everyone is Ex Andersen and B4 but they have also complimented and rewarded me for current accomplishments, so it really depends more on just what your credentials are on paper and how you really work, etc.  If your good then B4/Regional/Small firm will strive to keep you there, if you suck they will slowly start pushing your desk out the door.

Given your current situation I would FOCUS on getting your license First as this will be a better marketing tool for employment, if you still concerned try to secure an internship at the B4 to feel things out (be aware that they typically smoother interns in hopes of them coming back or unless your a total tool they won't extend an offer).

Also, normally the pecking order is from B4 down when people leave but I have known people to move up in the food chain when transfering firms.  I secured B4 positions prior to starting my career and also when I thought about leaving my old firm and actually I got offers as a staffer (3rd year) from the B4 even when I was a senior at my old firm.  I personally think the exposure level at a local or regional firm is better given you get your hands on many different areas of the audit and not just audit PPE for a large client for three weeks.  I have always looked at PA as being sort of an additional extension of schooling (post graduate school) so try to learn the most and leverage the experience for a private job.

What constitutes a person sucking at their job? What determines if a person is doing their job well?

I have never worked for the Big 4, but I have heard stories of people working around 60 hours per week during the non-busy season and up to 80 hours per week during the busy season.  I have also heard it widely varies based on the client.  But overall, you will probably not put in as many hours in a smaller firm as you would with the Big 4.

Also, don't expect much in terms of the salary at the Big 4 compared to other firms.  If you work at the Big 4 you are doing it for the intangible benefits, namely getting their name on your resume and building that career network through the clients you service. 

If you are ultra career motivated and willing to put in a lot of time for a few years I would go with the Big 4 / large firm.  If you want more of a work / life balance I would check out a mid / small sized firm.   

What constitutes a person sucking at their job? What determines if a person is doing their job well?

I thinkg poor performance reviews and really its just someone who isn't fitting in.  Usually the first to go are the ones that just don't catch on to auditing/tax or someone who doesn't put in that much effort into work.  They want someone who can communicate, reflect the firms image positively, do the work under budget, and gets along with everyone.  I just used the words sucking instead of writing an entire paragraph on what would constitute someone getting fired.  It think for the most part its obvious.  I think if you do a good job or are performing well people will let you know, I have always noticed that managers like to groom the people that they want to stick around, they get a little extra attention in different areas.  Of course this is my experience both visually (by observing) and through praise.