I assume you are looking at the Z5 3.5-18 because the Z6 is the 3-18. I have one of each. My Z6 is the BT and the Z5 is just the standard plex reticle. One of the interesting features of the BT for Swarovski is that it is limited to a single turn. That is fine if you don't need more elevation than the single turn will allow. If you are looking at shooting past 800 yards use the Swarovski ballistic computer and find out if your round will make the shot with the single turn of the turret. One way around that is to put a 20 moa mount on the system, but with the limited Swarovski elevation range you will be near the bottom of the range. So, take a look at how far you will be shooting, determine your true muzzle velocity (measure it) and decide if you will be able to get where you want with the single turn of the turret.
Having said that, I love it. I have the Z6 3-18 BT Plex on my FN SCAR-17s .308 and it puts the bullets right at the range I want them. Since the SCAR has a 16" barrel I don't expect to be using it regularly past 700 yards. And that is just about where the muzzle velocity puts me with the scope and the BT. I have a Zeiss on my Savage Model 10 .308 because I want the elevation to take me out to 1000 yards. So, if you do the math and you can live with the elevation limits, you won't find a better BT system.