What's the difference between iPad's a-GPS and iPhone's GPS?
The iPhone also has Assisted GPS. The iPhone page you pointed to only has abbreviated versions of the features; when you check the actual "specs" pages for both devices you get the same info.

Both get real GPS data (assuming one of the 3G versions of the iPad, the wifi-only versions have no GPS). The "assisted" part just means that it also uses cell tower locations and wifi networks it can detect to help narrow down your location, usually much faster than it can get a full GPS lock from the satellite signals.

In real-world experience they both work well, but both have fairly small GPS antennas. If you want really solid turn-by-turn directions while driving, an external GPS antenna is definitely helpful. Tom-Tom makes a pretty sweet combination mount, charger, and boosted GPS antenna.