My preference from my own perspective - Cubecart vs OsCommerce - is for OsCommerce.
The reason is that, although it may take a while to get a handle on working out what to do, how to edit and adapt it, change the layout, change some functionality and options, etc., it is in my opinion the one that DOES offer the most functionality.
You do have to be either PHP-savvy or prepared to learn some PHP.
It's not a quick start option, but it's the one for which there are many, many community-based add-ons, and for which you can create your own to suit your specifications.
Drawbacks: steep learning curve, and the OsCommerce community is not as active as it once was with regards to giving helpful advice. (This may change again.) Oh, and you should keep the little footnote stating that it's OsCommerce, but since you can change the appearance of this as you wish, and it can be blended in very tastefully as well as acknowledging the source code, I don't consider this a problem.
Pluses: Many, many questions have already been asked in the community forums, so that a thorough search may well elicit the answer you need anyway; there are many add-ons; you get the entire source code; you can make the thing look pretty much as you want (not just an offering of "skins" or "templates" - you get to play with the whole look); you can use an add-on for importing and exporting new products, as long as the product code is entered; you can enter manually; you can tailor it to particular postage costs per weight of total order; you can tailor it to the payment options YOU want to offer (including your merchant payment gateway)... Also... it's free. Completely. Completely free. You are free to change anything you want. Add-ons don't cost anything. There's no extra "special" version with more features. It is just... free.
It's not ideal, and it's not the easiest of options. I can see why other carts such as Cubecart are popular, as they are simpler to deal with and can look so much easier when you have to change something.
Functionality to my specifications and control in my hands were two of my major concerns when deciding on OsCommerce. Cost was another factor.