So I wanna lay down some maths, I've not gotten to deep into this but, per hour from grinding instances is about 150k EXP an hour at high 40s, if it takes you ~2 minutes to complete a FATE, and you get ~12.3k EXP per FATE, you would need to do 12.19 FATEs per hour to get 150k EXP an hour, now in theory at 2 minutes per FATE you could get 30 done if you could travel fast enough to the FATEs, you'd earn ~369k EXP per hour, BUT from personal experience, I've seen it take up to 10 minutes for a new FATE to pop up on any point on the map, some times they will pop faster, some times they take their sweet time. So if it takes ~5 minutes per FATE to pop up (excluding the chances of 2 fates being near each other or up at the same time) you're only gonna get ~12 done per hour if you even make it to the FATE in time to get gold.
unless someone can point me to solid data on FATE timers, FATEs are unlikely to be faster EXP then instances if you have a 4 man party for spamming instances