Non-zip suits are more flexible than the zips because they having nothing on the back to disturb the suit's elasticity, but when you take into account the actual range of motion most people use when surfing, the differences between the two are probably negligible.
Non-zip suits also prevent water from flushing down the back by not having a large entry running down the spine, but if you get caught inside on a wave with enough power, you're going to get the full flush through the neck anyhow.
Besides, a great deal of zip suits all have a batwing, zen zip, or someother element of design that gets pulled over the head and allows a zip suit to block out water and prevent flushing in a manner quite similar to a zipperless suit.
Granite wrote:Zip or no zip isn't imporant, main thing is correct fit for you. Try a few different brands as sizes vary greatly, best not to target one type and rule out others that may suit you better. In saying that, avoid Rip Curl.

Personally, I own the e-bomb chest zip and I love the thing. Things can go wrong with rip curl suits, but as a company, they were very good about their warranty. The liquid seam tape on the inside of my suit was defective, causing all sorts of leaks, so I took it back to the shop where I got it. 10 days later, I had a brand new suit. The guy in the shop said that last years models had defective taping but it seems as though things have been sorted out for this year.
As for getting in and out of the suit, it can be difficult the first few times, but after a while you learn to houdini your way out without any trouble. Wearing a rashgaurd can also help to keep the suit from suctioning onto your body so much, and makes changing go much faster.
Last but not least, as Granite said, correct fit is everything. If the suit fits perfectly, it shouldent matter how the suit zips up.