SkyTrace.
My experience with NFC (as a renter) was a disaster. 50% of the time I turned up, the aircraft wasn't available (away over night with someone else, or gone tech) despite my booking. The other 50% of the time I'd have to wait 1.5hours + for it to return from the previous flight, and was told to come back ASAP as it was already running late!
That's about 3 years ago, but from what I hear, not much has changed in that regard.
I'd go with SkyTrace who I've heard good things about, but have no first hand experience of.
dp