sub facing up or down

?

There is no difference sonically between having the magnet assembly inside the enclosure and having it outside.  The decision to mount a speaker like that is purely cosmetic.  The key to proper operation has been posted in this thread and elsewhere, but here's my summary:

1.  The size of the enclosure must be reduced to compensate for inverting the driver.  You must reduce the gross size by TWICE the displacement of the cone since you have not only removed the magnet assembly from inside the enclosure, you have also ADDED the volume inside the cone to the enclosure internal volume.

2.  Reversing speaker polarity is necessary if you want to maintain its electrical polarity in relation to the rest of the system.  If your amplifier or crossover has polarity control, you don't need to worry about this.

3.  The speaker is MUCH more subject to external physical damage when it is sticking out into the air than when it is hidden inside the enclosure.  Be aware of this when choosing your mounting location.  Also, you may need to weigh doen or bolt down the enclosure depending on how heavy the magnet assembly is.

4.  Since the cone (the speaker radiator) is now outside the enclosure, it effectively moves forward in relation to the enclosure baffle.  This can have an effect on time alignment and on acoustical cancellation effects, although in sub-bass frequencies below 100 Hz it may or may not be noticeable.

5.  If you are thinking about mounting your woofer sticking straight UP, then you must choose a woofer designed for down-firing applications.  Otherwise you can easily exceed the excursion capability of the speaker assembly and fry your voice coil.

If your speakers are mounted with the voice coils hanging out in the air, you CAN'T hide the wires.  This is another drawback, I suppose.  The only thing you could do is try and make them look nice.