Hi Xenocidial Maniac,
I recommend you that you should go for the army you feel more comfortable with, or the ones you enjoy more the period. This way the enthusiasm of collecting it and painting it will keep high and you will finish your project.... have few games and go for the other

But I give you also some reasons and comments,
About your gaming group...Depends on the people you usually play should be good go for romans or for helenics. If your gruop like to recreat battles then the choice will depend on the other armies of your mates. Romans have a good position in this way because Imperial Roman use to face a wide variety of different armies.
About the army list...In this way, I recommend you go for Alexandrian greek because once you build up an army is easy to buy a couple of extra units (like elephant, etc...) and build up another list like Succesors (diadoc wars). Also with a little effort you could expand your figure collection and have some list like classic greek, Pyrrus army, syracusian, peloponnesian or even spartan quite easy.
Roman fashion figures are more rigid to grow your army into new armylists.
About the warfare & tactics... Alexander y Roman are very different player style. So I recommend you go for the army that fit your style of game better.
Alexander army is fun because they have to move phalanx, and that is not easy in warmaster (neither in other warmges too!). So it is a challange for you as a player. You also need to combine this arm with a scarce but powerfull cavalry, so winning a game should need some experience in warmaster game, some luck and quite strategy. Due to this I recommend you go for Alexandrian army if you have experience with warmaster historical or warmaster (fantasy). Anyway be prepare to loose some battles until you get use with the different arms of your army.
Roman cohorts are maybe more easy to play with, more defensive and more reliable than phalanx. So I recommend it to you if you are new in the game. Anyway, playing with Romans is always very fun and pleasant, also there are plenty of films out there that could motivate you to keep painting and collecting such army!!
About the figures available...Depends a lot on your taste and the money you want to invest.
There is a catalogue here made with the intention of orient and choose figures, talk about the quality and utility of them, size comparison and also as a painting guide. It is intended as a living catalogue grow on the experience and collection of all of you. Maybe you could collaborate or benefit from it
http://www.forum.specialist-arms.com/index.php?board=63.0http://www.forum.specialist-arms.com/index.php?topic=4117.0http://www.forum.specialist-arms.com/index.php?topic=4118.0Anyway and focusing in your armies, here it goes my suggestion,
RomanIn my opinion the best figures are Perrin roman army, but I am not sure if you could buy it actually.
I would go for Steve Barber romans, as core units of the army and use Magister millitum or Pendraken to fill the gaps like cavalry or units to be differenciate quicly in the battle filed like veteran legionares.
There are other manufacturers like Irregular (lovely cavalry), old glory (nice detail), baggage trains (for allied troops), Van Dyck miniatures (beautiful bagages and wagons ideal for markers, command bases or encampents for scenarios), or Newline designs (to give variety)
AlexandreIn my opinion the best figures for phalanx are old glory ( I should upload some pics in the catalgue in the following days if you do not know the figures). They are lacking of Sarissas, but you could get them in pendraken [PEN] GRE13 - Pikes 30mm long 0.7mm wire
Again Magister millitum, pendraken, New line designs or Irregular will give persian allied light cavaly, light infantry allies, helenic cavalry and the other units to fill your armylist with your personal taste.
Have fun diving through the manofacturers and finding the figures that best fit your way of painting.
About your painting style...If you do not mind to paint a lot of units very similar go for Romans.
If you like to have varity and you loose interest in a project involving to paint the same figure over and over again, then go for Alexandrian army.
Hope my thought helps you, keep us inform of your choice and share your painting progress and games around!!