Ok, I got an iPhone & iPad, only I use two different libraries on iTunes. I got Google installed on both devices (plus desktop). But for some reasons only part of my contacts are on the iPhone, while all of them are on my iPad. What the simplest way to sync (and merge if necessary) these between devices? iCloud, Google (chrome/calendar/gmail), bluetooth, other app?
You can use the CardDAV protocol to sync your contacts. Here is an explanation how you can use it: Syncing contacts
There is also the possibility to use Google as an Exchange account. You should be able to find the instructions on the interwebs (I fear, if I post a link it will be in german language)
If you also enable the Google account also on your Addressbook application, merging them should be easy (depending on the application). In most cases, you just have to drag and drop the contacts to the corresponding Google folder.