This is a quick guide that might help you decide between the 2, please note that this is my personal view, I have not tested both side by side on my favorites single tracks and based this on specs and from my experience working for Luna as tech support.
FRAME:
Both have similar enduro frame set up, both in carbon, both from the same frame manufacturer. The shock mount and linkage is the only difference but it works pretty much the same way.
The Z1 is available in Galaxy black only
The X1 is available was available in matte black for the first batch but now it's in Galaxy black .
The Z1 does have water bottle holes, the x2 does not.
The Z1 in medium wights 62lbs (including all components)
The X2 in medium weights 54lbs (including all components)
BATTERY:
That one is simple, they both have the same 14S 5P 17.5ah 48V LG or Samsung battery, Both cells perform the same.
COMPONENTS
Both the Rockshox x2 and the Z1 share the same components. Only the X2 DNM has different fork and shock, brakes and tires .
MOTORS
Now this the biggest difference
But both have throttles both have torque sensor too. Motor motors are both made by Bafang.
The Z1 has a larger motor with 1600W, ( adds about 8lb more total) but being larger means that you can literally run hard from full to empty without ever getting the motor to overheat (more thermal mass). Perfect for someone how does not plan on pedaling at all on Sundays. The Z1 motor also sits lower to the ground and has a bigger chain ring (you can't go smaller than 42T due to the 130 BDC pattern) so it's not the best for super technical enduro trails where you need to climb high mountains and clear logs and rocks and such. The Z1 motor is also noisier than the X2 motor because it does not have the PEEK gear that the X2 has... Now I'm not saying a PEEK will never happen on the Ultra just for now the Ultra is as is! The Ultra does not have any power upgrades but it does let you program it like our beloved BBSHd motor, you can tune the assist and throttle to your liking with a basic programming cable and windows laptop (or our luna black box if you are lucky to have one). No speed limit so you can go 30mph + depending weight, wind, tire pressure and such.
The standard powered X2 has half the motor in size and power (750W), lighter, better ground clearance and a smaller chainring to help you climb. The X2 is made for the true enduro trail rider, the kind or rider that will pedal along with the motor and use it not to get carried to the top of the mountain but to pedal like superman to the top. Does not mean the X2 cannot move you without pedaling it just means that if you lug it for too long of course you will get it hot and then the controller will start to cut the motor.. But with the stock 750W controller it wont get too hot, no worries there! Oh it's limited electronically to 28mph on the stock controller and you cannot program it yourself, there is nothing you can do to change the behavior of the motor.
On the X2, you have the option to get the Ludicrous V2 controller, to get even more power.. up to 2000W+ ... yikes! But the motor is small so it will overheat eventually is you ride it hard and the power will drop.. See this power option as power on tap to pass someone on the trail but not to ride full throttle climbing Mt Washington, you wont get far, that is the best explanation I can give you! If used responsibly it's a pure blast. Now out of the box the controller will work but you may have to use your phone to tune it to your liking, adapt it to the type of terrain, you might have to dial it down a bit too if you want better battery life.... And in order to do so you will need an Android phone to use the luna VESC app to tune it VIA bluetooth. Click here for more info. The X2 with V2 does not have a 28 mph speed limit but still you can't go 50mph on this thing! Even if you don't need the extra power the V2 ludicrous still offers tunability and that is a huge plus, you can change a ton of parameters using your Android phone (sorry Iphone users). If you don't have an Android phone then do like me and buy a cheap Android phone just for that, it's totally worth it.
So if you are looking for an all round bike (all mountain and also tarmac) that you can use almost everywhere and enjoy "not" pedaling sometimes then the Z1 is for you. But rest assured the Z1 also has torque sensing so you can enjoy the same superman feel as with the X2 while pedaling.
If you want something that competes with the Specialized Levo and such, want the lightest bike with the best ground clearance and need a high powered torque assist bike to go through the gnarliest ebike trails (and like to pedal along) then you can opt for the X2.