there's different things you have to consider with both choices though.
if you give it to z1 think of how long do you think the actual swap will take them, or how long it will take before they actually
get to your
car because i've heard they have quite the line-up, especially after z-nationals last weekend. and also is 8k the base cost of the swap or will they add more should they need more parts along the way (ie new motor mounts, etc..)
or if you do it yourself would you even have the slightest idea of what you're getting into? like everyone said, the vg motor has not always been the most friendly especially when you're taking the whole thing out. if you did it yourself no doubt you'd need to know what you're doing but also you'd need to know exactly how much to modify from the vg30de to the vg30dett, because the differences are a big deal.
if you know what you're doing, no doubt you should do it yourself, you'd save a shit load but if you don't, you either need to have a shit load of patience or send it to z1 because they may charge a lot, but they know what they're doing and can do it right.