IMO, catgirls are a harder route, and gals are the easiest one in general.
The main issue with catgirls is that not only do they have very low max HP, their armors are bad as well - less protection, harder to unlock, overpriced. In the early game stealth works and catgirls are great. Later, every enemy will have maphack (also known as sniper-spotter for some reason), so you WILL get shot and grenaded. Catgirls just can't survive that. They are great specialist units, but you can't have an entire squad of them, unless you want to have half of them dead every mission.
Catgirl outlaws you get from the cat path are 30k apiece. Doesn't seem like a lot, but it really adds up in the early game, especially since they are much easier to lose than gals.