rt vs oflline export Which is better ?
Unless you need external processing, I would say it is better you use offline export, which is faster.
Offline export is as fast as possible, as slow as needed; usually faster than realtime. This works by decoupling from the soundcard. As @agus.terol already mentioned this is the preferred way.
Realtime export is intended for situations where you have external analog hardware part of the session (via send/return pair), and the soundcard is required. The downside is that, like normal playback, dropouts can happen when relying on I/O.
PS. Alternatively once can also bounce the external gear once (and record it).