they will complement each other... in any case, as far as apps:
in iTunes you will see 3 categories of apps (I'm leaving out iPod touch):
1) iPhone apps
2) iPhone & iPad apps (when app shopping, a + sign indicates one app for both devices)
3) iPad apps
the first time you connect your iPad iTunes will transfer all apps in the first two categories to the iPad that are compatible.
whenever you download new apps, whether that be on one of your devices or through iTunes, the next time you sync either device, iTunes will add that app to the other device, if it is compatible.
in other words, you'll have to do some manual managing of apps or you will have more duplicates than you'll use...eventually you can needle out which ones you want on the bigger screen and which ones you want with you wherever you go.
also of note: category 1) iPhone apps are run in a window that is iPhone sized or doubled (which doesn't look great, larger pixels, but almost fills the screen). While they are compatible, and you are not charged again, they are rarely useful on the iPad, some apps you may find yourself purchasing the additional iPad version of that app.
nice thing is, as long as you sync regularly, your contacts and calendars will be up to date on all your devices (iPhone, iPad and PC/Mac) (as long as you have it set that way in iTunes)
Of course, this is assuming you will be using your one iTunes account. If you set up the iPad on a different iTunes account, then all this is bunk.