I have been living in the world of iPhone / Android for quite a few years and have probably the best solution for your contacts sync between both phones (and it goes further to include calendars and desktop contacts/calendars as well). The key is to use your google account for your contacts and calendars. This will take some work the first time you do it, but it is totally worth it if you plan on cheating on Android when you need a phone that fits in your skinny jeans.
With Android when you log in with your gmail account everything just works, contacts, calendars, etc. With the iPhone you have to set up your gmail account as an exchange server so it keeps everything sync'd between the two. This sounds harder than it is... Here are some directions on how to do it:
Set up Google Sync with your iOS device - Gmail Help
Once you do that your contacts sync straight over. I do not remember if I could merge all of the contacts off of my iPhone into my gmail contacts the first time I did this, I might have had to manually enter them. There are other options out there, you can export your address book into a .csv file, you can do this from and iPhone or whatever desktop calendar program you are sync'd to (google your specific set up ... export to csv). Next you want to import that CSV file into GMail:
Importing CSV files - Gmail Help . Once you are in the gmail contacts you can clean it up by removing duplicates etc., this is where my OCD really kicks in, you have to have a clean contacts list.
With this set up you have to think of your contacts list as not just phone numbers but also emails and addresses, it brings it all together really well.
To get a little more nerdy now that I am on a roll, I actually use a "family gmail account" address book as our address book / calendars for our entire household and it is sync'd to everyones phones and computers. You can go this route if you want to keep everything centralized, it really works out well for me as I can make sure everything is always updated on all of the phones (we have both android and ios in my house) and computers (macs, chrome book, and one damn stupid windows machine at work), without even doing anything.
The best part about this is that you do all of your contact management in your gmail account. When you add a contact in gmail it is instantly updated on Android and iOS. You can also add a contact on either phone and have it show up everywhere as well.