I am with the RAC and personally feel that they are generally very good...
However... They both have fair usage policy and are IMHO very much better than Green Flag...
That depends where you are in the country

When I worked for the garage in Basingrad we were the main Green Flag agent. My average attendance time was 17 minutes and my roadside repair rate was 97%. If we did work for the RAC it was normally only passed to us after the poor person had been stranded for more than 4 hours while they tried to get a patrol to them

Green flag do not have their own patrols, they are all local garages, so the wait is normally shorter 
However, where I live now GF aren't particularly good but the RAC have been the only time I have called them
(well... Arrived in a little over an hour but did come from 40 miles away
)
I have 2 very poor experiences with Green Flag...
The first was when I had them come to look at my old 1.8TD Mondeo, which wouldn't start... When they came out, they fiddled about under the bonnet for a while, tried to start the car (which failed) and asked where they wanted me to put the car... I had a quick fiddle and found that the fuel primer diaphram had split slightly causing an air leak, manage to prime the engine and start it... Green Flag chappy was not impressed...
2nd Occasion was in Anglesey, on holiday with my Vectra 2.0 DI, which refused to start after the leak off pipes had been replaced.. The Green Flag prat turned up (with his bit of stuff in tow) and started implying that the diesel system was a closed system and that he wouldn't be able to get it started either... After a lot of tutting and indicating that it was going to be costly to get it started he said he would try a tow start, which, started the engine... He then undid the tow rope, said that it would be fine and buggered off so quick I didn't have the time to get back in the car, at which point the engine died and wouldn't start... Leaving me stranded 2 miles away from where I was, with nothing around me (or phone signal)... Strange thing was, when I did the same thing with another tow (and a little more fiddling, easy to spot problem) it ran fine...