I've been a big Zen Cart fan for a while. I did a project with Magento for a big brand, and it was an interesting experience. It was a nightmare for me, since they wanted out of the box customization, which were not easily available through Magento's module.
I've utilized PrestaShop, and talked with the CEO and their top developers, their system is very impressive, but I have not yet had the opportunity to attempt to implement a live version.
The only downside I see with Magento, is that it requires so much resources to get it working properly. Like harrymouni stated, you need a dedicated server, and will be coughing up a minimum of $100 a month to run the thing. The one thing that really pisses me off is they are using 2 engines to process/run their shopping cart. I feel the added javascript engine just lags things if you don't have enough ram or system resources.
I like barebone systems, like Zen Cart, to get started with projects. Yeah, the backend looks shitting, but it works. Prestashop seems pretty easy to setup, but then again, I don't know the extent of the customization you are looking for in the future. Also, remember, Magento programmers/developers are going got cost you, since it's a fairly complex system under the hood, so if there is no module to do what you are looking to customize or what not, then your will end up paying, and you need to go for quality.
It's like buying a high end car versus a Geo Metro, One is going to cost you alot more in the end, but it will last longer, the other will get you places, maybe not in style, but it will work, and the parts are cheap.
Good luck bro.