HP Slate runs flash. along with other tablets. no need for special apps. code once and run anywhere
The HP Slate doesn't have a firm release date or even confirmed hardware specs. At best it will be a typical Win 7 netbook with a touchscreen in the place of a keyboard and trackpad. That means it will be slow, bloated and feature a UI that is meant for the wrong input method. Many of the apps people will be running on the Slate will also be designed for desktops and meant for machines with more CPU and RAM. It's a mistake for a Windows machine to lack a mouse and keyboard.
The "game changing" sauce that the iPad is bringing to the market is a mature UI and development platform for touch based input along with standardized hardware. I guarantee that your actual experience running apps on an iPad will be much more responsive and intuitive than on the HP Slate.