FB wizard will run a bit higher, but slower. GoD will run a bit lower but faster.
If you’re going solely for max XP efficiency, this difference doesn’t matter much from that perspective.
There is one important difference though: even if the XP/hour is the same, running lower but faster means more drops, shards, and gem levels per hour.
If you are planning on farming 3k paragon, then a little higher but slower is fine. At that point, maximal XP efficiency is the more important goal. You’ll get enough drops and gem levels anyway, it doesn’t matter if you get less than you would with GoD because you’ll still be saturated on augments and gear.
If you are planning on playing a bit more casually, that’s where running faster but a bit lower could be helpful. If you are only planning to play until 2k paragon and want to push multiple builds so need multiple sets of augments, running a bit lower but faster means you’ll have more augs to use for the same amount of XP gained.
In my experience, there’s a bit of a range where you can run higher but slower and where your XP efficiency doesn’t change by much. You can pick something in that range and you’ll still be within 5-10% of your maximal XP/hour. If you go too much below that range or too much above it, your XP efficiency tanks, but as long as you’re somewhere in that range, it shouldn’t matter too much for most players.
Another thing to consider is that if you run lower but faster, you really need to be efficient in town. The faster your rifts, the more important your town efficiency becomes, since it shifts the ratio a bit from time spent in rift to time spent in town. If you are running lower but faster and you tend to dally in town, it can really tank your XP efficiency.
If you know you are someone who likes to dally in town, it may be better to run higher but slower. That forces you to spend more of your time in rift, so even if you do dally a bit in town, it hurts your XP efficiency less than it would if you were running faster rifts.
So it’s a tradeoff. More augs/drops/shards but you need to be efficient in town vs. less keys spent, more time running GRs and less impact from dallying in town. Pick your poison. It probably won’t matter much in the long run from a purely XP standpoint.