I hope you don't mind me sharing the photo above from your blog, Melina. I get this question a lot, so I thought the answer deserved its own post.

Eliot Coleman did some research with different fabric weights and found that, counterintuitively, lighter-weight fabrics are actually better at protecting the plants underneath. The heavier fabrics block more sun, and it's really the sun-concentration effect that protects your plants, not the night-time quilting effect. So, go for the thinnest fabric, which in our experience (like yours) has been agribon-19. Yes, it might not last quite as long, but it will do a much better job in the interim.

I haven't done as much experimentation as Coleman has, but I did originally begin with thicker fabric, which I used on cold frames instead of quick hoops. I've since retired that cloth, though, because even though it doesn't tear as easily, it blocks way too much light to leave on plants 24-7. If you feel you need an extra layer of protection (which Melina shouldn't, but which northerners might), then a layer of greenhouse plastic on top of the quick hoops during the coldest part of winter will help. But you'll have to remove plastic during hot days since, unlike row-cover fabric, plastic won't breathe!

Finally, I should briefly address Nayan's comment, since she complained that her row cover fabric "didn't even last from spring into summer. They literally crumbled into dust." Row-cover fabric is fragile, but if you take care not to tear it and always put it away dry, I've found that the fabric easily lasts two years with no mending and three-plus years if you're willing to sew a little. We definitely have not experienced any crumbling into dust!