I guess since this is the Strategies & Tactics section of the forums, this isn't just some poll we could answer or not. So I'm guessing you want specific advice, reasons and tactics for doing it right? Well, I'm just lazy, so I'll give you just why.
So, this is a usual problem a builder encounters in-game. You make two turrets, Dragoon comes by and beats the crap out of it, turning it's HP to 1/low, then runs away while your teammates are chasing it. This gives you enough time to either repair it or make another one.
What I would do is repair it, because repairing it would take less time to repair then build, and since I already built it on my spot I believe is tactically effective (Thus thats why I built it there), I rather not build another turret in a less effective spot. Besides, the building turret gives any alien able time to attack and possibly destroy it if it comes back/another alien comes, making the choice useless. If my repairing turret does die before I fully repaired it, no problem, I can always build another one if it isn't SD/I'm not attacked by 5 other aliens.