Go with the Nova.
The Nova primes fast, has a easily adjustable (relatively quiet) flame and good reliability. The flip stop is nice feature too (takes about a minute to burn fuel out of the line so plan acoordingly to save fuel). Its hard to spill any fuel with this stove, unlike the XGK. It is slightly heavier than the XGK, but its a trade off for the features.
The XGK is also a good stove and comes with a windscreen too. The priming process is a little trickier and longer than the Nova. You have two options for flame adjustment: 1)OFF, or 2)BLOW TORCH. Its really LOUD too. Pot stability is good and is best for larger pots. A little care is needed when taking the stove down, so as not to spill fuel.
I don’t have a scientific data so I dont know which one is more fuel efficent, etc. From an end user stand point, I like the Nova much better