I think the best way to pick your first army is to try and find one who's troop choices you'll enjoy painting the most. If the troops are really interesting to you and you'll enjoy painting them, that means you're more likely to finish painting them and be encouraged to play games. I guess this doesn't apply if you want to play with unpainted minis though.
That is a very good point. But enjoying the painting side will lead to needing to find a second army much sooner. Find that right balance of fun to paint, right general tactics (horde/shooty/etc), and enjoyable lore. It doesn't need to be the max on all three, but if you really dislike one of those aspects you'll be very disappointed.