Difference between Eclipse: "Help->Marketplace" and "Help->Install new software

The Marketplace Wizard is a interface for several websites that provide different plugins. These websites are maintained in general by the provider of this site.

The "install new software" Wizard points to several URLs which provide p2 repositories to install any software you like.

Main difference
Marketplace: You can install the bundles (and only the bundles) hosted by the selected marketplace
Install new software: Add a URL and you can install all bundles the URLs repository provides.