The most common method is to sync both to Google. The best way do this on the iPad is to set up Google as an Exchange account. This will allow you to sync your contacts and multiple calendars. All you lose is note syncing.
If you are already doing this and only getting 90%, all I can suggest is turning the Calendar syncing off in the Account settings (let it delete everything from the iPad) and then turn it back on.
I've also had problems in the past with events created on Google if they had more than two alerts set. Calendar will only handle two, and it rejected an event with more. That even happened when I was syncing with MobileMe; which gives you and idea of how long ago this was, so the issue may or may not still be present.
As twerppoet very correctly posted, Google Calendar is the single best multi-device system for smart phones today. It allows both read/write and read only access to the calendars. Which allows both individuals and companies to create calendars that can be sunk to 1 or 1000 devices.