So picking your army.
I'm not going to say which you should pick - for reasons that will become quickly obvious, but rather give you some things to think about that may help you in your choice.
First - Period or Free?
Are you planning to play in a narrow historical period or take on all comers? Impetus will happily allow you to play armies from the Biblical Era to the end of the Middle Ages and Early Renaissance, and it will allow you to use New Kingdom Egyptians against Medieval Burgundians if you so wish. That doesn't mean they are equal however, as armies from different periods will have wildly different fighting styles and capabilities, and unlike Fantasy or Sci Fi armies, they have those styles and capabilities for real world reasons which are mostly to do with their "regular" opponents. As a general rule the later armies are smaller and harder hitting, so if you are looking for "killer" armies Late Medieval ones may well fit the bill (pun), however the earlier periods are often far more interesting AND more fun to play. Again this is very much a question of your gaming group. If there are already a core of period armies it would be best to choose something that fits in, DO NOT PICK HUSSITES*. There is a good core of "Classical" Period armies in my area, Greeks and Romans etc, so they are a great place to start.
Second - Competitive or Friendly?
Yes we are all friendly around here, but there is a little bit more to it than that. (If you are not a competition player please don't get put off competitions by what follows, just be aware.) The problem with competition formats is you need to play three games in a day, plus time for lunch, fags, cuppa's, a bit of shopping etc. It follows that you have usually about an hour and a half to three quarters to play each game, from opening the box to packing up. Also to "do well" in competitions you need to score well in all three rounds - narrow wins or draws do not help. That means winning competition armies must be direct and to the point - you can't spend too much time softening an opponent up, messing with ambushes, flanks etc or trying a lot of finesse, you just need an army to smash the opponent in the face and shock and awe him into oblivion. The ultimate competition army is of course Swiss Confederates, who are so single minded and aggressive it is hard to believe the same people gave us Cuckoo clocks and Toblerone. Getting hit by a Swiss army is something like getting hit by a truck. The other side of the coin is if you are playing friendlies or not that interested in being a top scoring competition player you can pick armies with more finesse or that take time to wear an opponent down. A good example of this phenomenon in action are my beloved Persians. They lack any real punch so usually do average to poor in competitions, in fact I've only ever came top once using then and that was a fluke. On the other hand they're a great army for club \ shop games where you can take a bit more time and enjoy the game more.
Lastly, and most importantly, do you feel a connection?
Eh? Ok what I mean here is that you need to feel some kind of connection to your army, know a bit about them, or at least find out a bit about them, learn why they fought in the way they did. Impetus armies are a major investment in time and effort, not to mention money, and if you are going to get to the end of that building stage and get the return in enjoyment it helps if you can identify with "The Lads". I'm not suggesting you dress up as Hannibal (you can if you want) but you will get more from the whole experience of building and playing with your army if you have an affinity, no matter how daft or indirect. All my armies are chosen because I feel something about them interests me. I like the religious pigheadedness and modernity the Hussites represent, I like defending Richard III as the brave and savvy King not the panto villain Shakespeare turned him into, etc etc
* unless of course you are playing with armies from Late Medieval Europe in which case wheel them on, OR if you just take pleasure at watching 99% of opponents stare across the table at you in perplexed puzzlement. Hussites are almost unbeatable in competitions. They are also totally useless in competitions because they rely on your opponent smashing his army to pieces on the front of your wagons. As soon as they realise this you may as well call it a draw and pack up.
Next - a couple of examples of what armies to do, and possible what not to do