Which technology do you propose to use for a new DC fabric? It is a vPC migration project.
What are the advantages of one technology over the other?
Currently ACI is not an option for us.
You mentioned that ACI is not an option for you at this point, fact which rised a question in my mind: does your migration concerns N9K switches? If yes, then FabricPath is also not an option for you, as Nexus 9K switches does not support this feature.
Coming back to your question, the answer is not so easy to find, as you have to keep in mind all your network requirements while choosing your Data Center Fabric.
As a further matter, although both overlay technologies offer relatively same outcome - highly scalable layer 2 multipath, you will have to take in consideration the pros/cons for both protocols.
I would recommend the following documents/presentations, which highlights the differences between these overlays, and will definetly help you in choosing the most suitable overlay for your network:
1. Ciscolive presentation BRKDCT-3378 - Advanced - Building simplified, automated and scalable DataCenter network with Overlays (VXLAN/FabricPath) (2015 Milan)
Here is the slide deck: http://d2zmdbbm9feqrf.cloudfront.net/2015/eur/pdf/BRKDCT-3378.pdf
For the video presentation, you will have to login (free) on ciscolive.com and search for the presentation under On-Demand videos
2. Book Using TRILL, FabricPath, and VXLAN: Designing Massively Scalable Data Centers (MSDC) with Overlays
here is the a short chalk talk about it: https://supportforums.cisco.com/t5/data-center-documents/chalk-talk-fabricpath-trill-vxlan/ta-p/3134732
3. Data Center Overlay Technologies (with focus on N9K)
One thing which you might want to consider is that VxLAN has some strong improvements over FP, which reflects the technology as an evolution in terms of DC Fabrics:
Hope it helps!