First things first: The naming of the site is a bit misleading, as these are very historic places with multiple different names and spellings:

• Harran was known to the Romans as Carrhae. It is here that the Battle of Carrhae took place where Crassus and his army were slaughtered.
• Sanliurfa was known for most of its history simply as Urfa. The Turkish authorities added Sanli (glorious) in 80s to commemorate the valiant efforts of the locals during the Turkish War of Independence after WW1. Effectively, everyone still calls the place Urfa. You may also know the town as Edessa, which is the Greek name and was for a short while a crusader state.

Both cities are referenced (or claim to be referenced) in the bible, specifically relating to Abraham. Harran was a temporary home of Abraham and his ancestors. Urfa calls itself the City of Prophets and the birthplace of Abraham. This view is disputed. Still, the most prominent complex in Urfa is the area around the fish pond with the famous cave where according to local legend Abraham was born.

I came from cold Malatya in Anatolia. It was delight to leave the cold Anatolian plateau and arrive in Mesopotamia. Urfa and Harran more so no longer feel like Turkey, let alone Europe: This is the Middle East, After my mandatory visit to Göbekli Tepe, I dropped my luggage off at the hotel, a gorgeous historic town house in the old town. I felt like a prince.

In the afternoon I went to Harran by dolmus. The ruins are massive. There are several prominent structures left, most notably, the walls, a gate, ruins of a church/mosque and a Roman castle. The most famous structures are the beehives on the Eastern side of town. Unfortunately, the state of preservation/presentation is poor. As are the local inhabitants: Harran was the only place in Turkey where kids were begging.

The next morning, I visited Urfa's main sites. The old town and bazaar area are nice. The area around the fish pond is the obvious jewel with Abraham's cave and the mosques. Overlooking the area are ruins of a castle (in fairly terrible state, supposedly a bit dangerous and most certainly closed to visitors at my time of visit). Last stops on my visit were the Necropolis and the two stunning museums in Urfa: the Mosaic Museum and the Archeological Museum.

OUV

Urfa without a doubt belongs on the list. Temples, old town, necropolis, castle. Amazing.

Harran, too, is a strong contender and probably as detailed by Els the more historic place. The beehives are unique. The Roman ruins are plain massive. But apart from the walls, most of the site is in poor state of preservation/presentation and should be polished up a bit.

I also don't understand what groups these two sites together and would favor separating them. Both on their own would be great additions.

Getting There

Urfa is a major Turkish town with bus and plane connections. The bus terminal (otogar) is on the outskirts of town. Seeing cab fares are low, I would simply take a cab to the town centre. Personally, I went from the otogar to Göbekli Tepe by cab. First things first. Urfa old town has small roads so it may be best to simply get off and walk.

To get to Harran, you can catch a dolmus. The online descriptions are not helpful if you are staying in town centre. You don't need to go back to the otogar or the main junction way North of town. The bus runs along Akçakale Cd. I waited at a bus stop in front of the Urfa City Museum along Akcakale. Note: You are getting really close to the Syrian border. There are travel warnings for the region.

While You Are There

Nearby, Göbekli Tepe is a must visit. It adds to the story of the region being central in early human settlements and development of religion. It made me wonder, how continuous the religious importance of the area was. If you enjoy Göbekli Tepe, you may want to check out Karahan Tepe, too. I learnt about Karahan Tepe at the museum where the finds are shown next to Göbekli Tepe's. They don't pale in comparison. I would probably favor an extension to encompass both.

In town, the archeological and mosaic museums are a must visit, as you get to see splendid and massive Roman mosaics as well as findings from Göbekli Tepe. There is also a bus connection from the archeological museum to Göbekli Tepe it seems.