Sub vs. three way

No not at all. I think everyone would love a pair of speakers that go all the down to 20hz but those speaker are very very expensive. Adding a sub to monitors will give you fuller sound but bass at 20 to 80hz will be very directional. My monitors go to 70 hz (50hz at -6db) and adding a rel b3 made a world of difference (all for the better for me). Now I would like to add a second sub but most likely never will. On the other hand if you can center your sub inbetween your monitors you wont really notice the bass being as directional.

No not all the same. Essentially a three way speaker will not use the same design compromises as a small two way. A dedicated midrange should allow much lower distortion at higher playback levels and if done correctly can achieve much better dispersion than a two way (two ways tend to beam because they use a large 6 inch woofer right up into the midrange). Beaming means that the off axis response is out of balance with the on axis response and makes the speaker ill-suited for far field listening (where you hear a lot of reflected energy). Two ways are best when used in near-field configurations. Three ways are best when used far-field (they actually work in near-field too but are so big that it can be cumbersome to get them positioned appropriately for near-field)

In addition to the above problem, if you add a monster sub to help two way near-fields work in a big room then you will also run into compression issues with the midrange and tweeter - these will sound harsh as the small two way was never designed to play really loud (as it needs to do in a far-field application).

Shadorne hit the nail there. It all depends on the room. If your room is huge I would say go for three way with large drivers as opposed to small monitors and a sub. If your room is small, monitors with a sub is the way to go. I have monitors and subwoofer configuration in smallish room of 11'x17'x8'. The 4" drivers of my speakers are rated at 35Hz but the bass roll-off at around 60Hz in my room. The subwoofer really helps in fleshing out the bass and just like Xti16, this made a huge improvement to the sound of my system.

If your room is to the smaller side of things, I would highly recommend that you add a subwoofer to reinforce the bass of your system, more so since your monitors can only manage 80Hz. In reality they would roll-off at a higher frequency due to placement from wall boundaries. The quality of the subwoofer plays a part too as inferior subs with large drivers may not perform as well as high-quality subs with smaller drivers.

My room is 12 X 24. I'm considering what effect the long shape will have.