Well you're in the OpenCart forum so of course you're going to get biased answers mostly.
I for one can be objective though.
I tried both PrestaShop and OpenCart, and settled on OpenCart. It's a lot easier to work with in my opinion. The backend is more intuitive. I found it hard to navigate around Presta's backend. Also it's easier to modify the code. While i'm no coding expert, i'm one hell of a "copy and paste" kinda guy. Give me some code to copy and paste and I can do it in OpenCart in seconds now.

That most likely has to do with the file structure of OpenCart (thanks Daniel).
Also we have vQmod with OpenCart which means you don't have to modify core code anymore to add extensions. I finally wrapped my head around vQmod and have made several of my own vQmod files for my store (yay me). Not sure if PrestaShop has anything like that yet.
Finally the extensions here are a hell of a lot more affordable than they are on PrestaShops website. I think that's a big draw for OpenCart. In the end, it's mostly about the dollar, and if I can upgrade my store and make it a lot nicer than what it is for less, then I'm 9 times out of 10 gonna go with that solution, and OpenCart wins hands down on this one.
One thing I will say good about Prestashop though, is that company wise they are structured a lot better than OpenCart. They run it more like a business instead of an OpenSource hobby project. They have a whole team of employees who communicate with their customer base, and they even have yearly conferences in France for all their users, so if you live in Europe (or feel like paying an arm and a leg for a plane ticket) you can actually go meet and chat with them. I do like that aspect a lot.
Also, PrestaShop appears to be focusing on Mobile Commerce a lot now and have promised some default mobile compatibility with the script in upcoming versions. At the rate smart phone usage is growing, I think Mobile commerce is going to be the future. I'm not really sure where OpenCart stands on this because Daniel or Q have never really mentioned anything about mobile commerce, whereas PrestaShop has released a lot of stuff about their plans for it.