Fewer of the core functions are missing in prestashop like compare option. Opencart is little ligh weight than prestshop but from community point of view and documentation , prestashop seems stronger.
For my opinion, PrestaShop is more powerful. It has a lot of useful features, which are easy to use. PrestaShop needs less resources, and it works really fast. There is a large number of extensions, modules and add-ons. I found this comparison. I think it would be useful for you
In new opencart 1.5.5 the filter option is in core. You can assign product filter like color, price, range etc without any third party module. Some of the great features like product comparison, wishlist and product filter (1.5.5) is now core in opencart. So no more pay for 3rd party modules, really easy and work like charm.
presta small footprint new version is good and easy to add papal and one click installer in cpanel plus auto update
Dont get fooled by the "free" opencart! The core files are free, but small modifications that should have been inluded in the package is often very expencive. Yesterday i was looking for a way to mark my products with the status:in stock, in stock at supplyer and out of stock. The only way i can do this is to enable costumers to check out products thats out of stock (since the only way to customize a stock status is to edit the sold out status). The only usable mod i found cost from 20$ and up. And usually the language cost money, a slightly customized shipping module cost money, auto post new products to facebook cost money and so on. Dont get me wrong, i dont mind paying for other peoples work, but when a cart is FREE then you would expect small things like this would be free too.