Sub vs. three way

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.

BTW, bass from a subwoofer is no more directional than from full range speakers, i.e., it is not.

Sorry Bob I have to strongly disagree. My sub is approx 1 ft to the left of my left monitor and the deep bass is louder from the left side of the room. To me that is what I am calling directional. I am also sitting in the 'sweet spot' when I hear this. I had the sub on the right of my right speaker and the deep bass was louder from the right. My room dimensions are approx the same but I'm doing a 7ft equilateral triangle.
I wish I could center the sub but I cant with my setup