CubeCart all the way! Not only is it easy to use its as powerfull if not more than oscommerce. Skins and templates are also considerally cheaper! Check out my sig for some examples.
For mods you should checkout www.cubecart.org there’s loads of mods for CubeCart which are considerably easier to install than anything on oscommerce, you can also request a mod if theres something in paticular you are looking for and everyone in the forum will do their best to help you out without being condesending;)
As on_way_to_fame stated there are a lot more features to Oscommerce than just ease of use. Yes Oscommerec might be a little more difficult to initially get used to but once you are over that hill you’ll have gross amounts of addons, customizations, skins, and programmers familar with this program (just look at elance.com).
Since Oscommerce is the largest open source cart how can you expect this much flexibility with any other cart?