Given a choice of one or the other, I would definitely choose paracord over zip ties. Here are a few reasons...

1. Paracord can be untied and removed, zip tips must be cut and trashed (Moving camp perhaps, or expanding your shelter, or rebuilding after some damage)
2. Excess paracord can be cut off and used for something else, extra tag on a zip tie isn't so useful
3. Lashing beats the security of zip ties any day. I don't know how you would make a very secure tripod (for a shelter per say) with just zipties.
4. On the whole, a length/weight of paracord seems more useful and multi purpose (bow and drill fire, wrapping bandages, sewing/fishing with inner strands, leash for your dog, etc) than an equal length/weight of zipties. I personally don't have any zip ties whatsoever in my kits, but then again I am competent in knots and lashing. Here are 2 tips I've learned for those of you that might not be able to lash together poles for a shelter:
a) take that survival saw of yours and begin to cut a small (about 2-4" dia, 12-15' tall), straight tree at thigh height. Instead of cutting all the way through, leave a little "hinge" that, once the top of the tree falls, will still secure your new "ridgepole" to the thigh high stump. Strip the branches on the ridgepole and begin to cover will all that nice leaves/bark/branches/dirt that makes a good water and wind resistant derbris shelter
b) find a "v" in a tree or where a branch meets the trunk at an appropriate height. Place your ridgepole in the v (you can place a log/rock on top of the far end of the ridgepole - or bury it - to keep it from slipping out of the v) and build your shelter.

Hope that helps

Matt