They are both great Pokemon but I would go for Cacnea becaue it will evolve into a stronger Pokemon. Although it mainly depends on what you want to use the Pokemon for ie. which types of Pokemon you would be battling.
well i'm headed to norman but I kinda think ninjask really isnt that strong but the problem is that cacnea doesnt know absord but I could teach it secret power or bullet seed though
Ninjask is very fragile, but if you want a baton passer it works cause it can BP speed boost. Cacneas pretty delicate too; neither are defensively based
Cacnea and Cacturne aren't really that good. Low Defenses and low Speed make for easy pickings, especially by Slaking(s). Ninjask, though, has a solid Attack Stat and a remarkable Speed Stat. Dual Physical STAB and a wide movepool doesn't hurt either. IMO, go with Ninjask. It may not be the best choice, but it's a better one than Cacnea/Cacturne.
As Surf said, those choices aren't really good picks for Norman. Which Pokemon is your starter? The Torchic evolutions are always great choices for Norman (as Combusken and Blaziken are Fire-Fighting).
I would suggest catching a Machop, Makuhita or Shroomish and raising them to evolve. They all will have some great Fighting moves later on.
Remember Slaking has high HP and Norman likes to use Hyper Potions and such (from my memory, anyway), so you need a Pokemon with good attacks and Speed. Good luck
Like intergalactic platypus mentioned earlier, neither is great defensively, to say the least. Ninjask, however, can work great dividends for your team if you take advantage of its abilities and Baton Pass a speed boost to a Pokemon with good offenses.