I can't speak from experience, as I am merely a future trainee. However, I can say that from everyone I have spoken to, the NQ market is far more buoyant than the 'TC market', for want of a better descriptor.
If you have trained at a good firm, it is relatively easy to move between firms - however, bear in mind that 1) you might grow roots in a firm, and not want to leave, 2) the firm that you wish to leave and the firm you wish to join may have substantially different cultures/working habits, which you may or may not be amicable to, and 3) your qualification area will determine a lot of options (not that you should necessarily decide your qualification practice area based on this - just something to point out.
But, yes, in short, train at a good/top firm, and virtually all avenues are open (not to mention alternative paths like in-house/non-law careers).