Subwoofers - Front Firing or Down Firing - Which Sounds Best?

It depends on the driver. Drivers with stiff suspensions can fire downwards fine. Those with looser suspensions are better off front firing as they may sag mounted downwards and the driver becomes non linear. Given a driver that can go either way there is little if any difference. More important is that the driver have one edge right up against a wall or corner. Then multiple drivers and finally room control. The radiation pattern of the sub should match that of the satellites. Point source speakers should have a point source sub system. Line source speakers require a line source subwoofer system. A Line source sub will dominate a point source satellite and a point source sub will get lost under a line source speaker. 

@shakira - Nice response! Simple, clear and to-the-point! I like that...

I have one set that is downward firing (a pair of woofers mounted face to face (isobaric on a vertical axis, vented laterally) and one set that is forward firing and find no significant difference between them.  It is more important that you have two of them than which way they point, IMHO.

I like the design of the REL subwoofers. Rythmik would also be another option.

I bought a Rythmik F12G from an A'gon member a few years ago.  He told me something about setting it up that was spot on and relates to the earlier reference to time and sound waves. He told me to add a few feet when it came to any room EQ setup where the distance between the speakers/sub and listening position was  adjustable.  My room EQ automatically adds about 1.75' extra feet to the distance.  I played around with it and eventually landed on adding 8-9' to the distance between the sub and the setup mic.  Actually it's something like 8.3 - 8.7 feet depending on how I have the room arranged furniture-wise.
I have been been able to integrate - for lack of a better word - my single Rythmik sub between my MMGs through this added distance and it truly incorporates itself quite well.  The integration is MUCH tighter after the additional distance, the lower frequencies do a much better job of disappearing and melding into the music, and it makes for a fine two-channel listening experience (to me).  The sub is spec'd to extend to 14 Hz and is highly adjustable.  I set it at a 20 Hz extension (with high damping) for the Bach pipe organ music I occasionally treat my cross-water neighbors to, and it does a fine job of rattling the wall hangings (it that happens to be your bag).
I don't happen to have the room for a swarm in my listening room (that happens to double as my living room) nor do I particularly want one.  I'm quite content with what I ended up with after much research, adjusting and listening.  But you CAN achieve perfectly enjoyable results that sound very, very good with a single sub when faced with certain limitations.

I have a down firing woofer that I can just lay on its side, and experiment with positioning.  I've found that facing it forward helps it blend just a bit better with the mains, but I keep it pointed down for aesthetics.  While I doubt the phase means much at 30Hz, how the driver is facing relative to your mains at 100-200 Hz will change the integration.  Sure, your sub crossover may be set for 80Hz, but that's only its -3dB point.  It still will have some audible output at 150-200Hz.

And don't believe that "Sub is omnidirectional, so you cannot localize where it's coming from".  THAT IS FALSE!   The first part does not begat the second part of that statement.  YES, bass is omni.  It will eminate from the subwoofer in all directions equally.  But that does not mean that you cannot localize it from your listening position.  That's an entirely different thing!  The BEST is to have STEREO subs alongside your main speakers (or 1 sub in the middle).  This will expand your soundstage tremendously.  For me, having a sub off to one side makes me feel uneasy.  I can feel the pressure differential in my ears.