CS is based on an MVC Architecture while X-Cart is based off antique coding practices. CS has a plug n' play Addon system, in CS if you install an addon and don't want it you simply uninstall while in X-Cart the addons make a mess of the entire cart system because the architecture is drastically different.
Actually CS and X have almost nothing in common other then they are both located in Russia. The administration panel in CS is far superior to X, this and the points noted above are enough to choose CS.
And as if you could not find enough reasons to choose CS you would never find the detailed API and Developer Documentation at X: http://docs.cs-cart.com/