I think if you are talking about versatility vs. weight/space carry concerns, the paracord is going to win. Zipties are much lower on the multi-tasking scale. However, for BOBs and other longer term (read bigger kits) situations, a quality ziptie may be the way to go.

For starters, let me clear up a common misconception. For most quality zipties you can re-use them by using a small tool to release the catch. On to the job at hand. I've used zip ties to attach a brass grommetted nylon cloth to a pole, and despite fairly unbelievable abuse, they've held up over the long haul. It's taken literally years to break the plastic in what I would consider extreme circumstances.

They are easy to use, even with pretty cold/numb hands (don't ask!). You just have to steady your arms somehow and use larger motor skills to get it started. They can be tied with one hand and a little ingenuity. They won't flex as much, which can be detrimental or beneficial depending on the situation. They require no skill to lash down. They have a very high tensile strength. They won't rot. They don't care if they get wet.

They might care if they get really cold, but again, my own experiments bear out -10 deg. F or worse conditions under heavy wind loads. I will say, that when mine have failed after years of service, it's been in the winter!. The newest set I'm using are UV resistant and I'm using two in place of one... they've already outlasted the old set. They are FAST to setup.

I'd say from the one-handedness and no-thought or skill requried, they deserve a place in your kit. Real 550 Paracord is great, doesn't rot and is very versatile, and certainly you can lash with one hand. But can you do it one handed in 20 knot winds blowing your tarp in 10 below weather? Anyway, I'd vote for both. If I had limited space and weight, probably go with paracord. But I'm guessing that even over weeks, you wouldn't notice much difference between the two if you have quality lashings in the rope and quality outdoor (UV stable) zipties.