Bosch's tend to fail quicker, but are cheap. NTK's (the sensor) are very resistant to failure, but not immune -- but $$$$.
General rule of tumb: if you are looking through the exhaust pipe, the mounting point should be placed between 10 o`clock and 2 o`clock, mainly to avoid condensatation.
Evac would defintely skew readings, keep it as far upstream as possible.