Based on my evaluation at the SHOT Show, I would say that the Z5 scopes are a cut above the HD5 scopes in terms of resolution and contrast. The Z5 optics are essentially the same as the Z6 optics, but in a 1" tube, so the zoom ratio is a bit lower (less erector tube travel). That's not to say the HD5 scopes are bad. They are typical of Conquest optics, which are good and set the standard for $500-1,000 scopes.
HighPowerOptics is an authorized dealer for both Zeiss and Swarovski, and has both HD5 and Z5 scopes in stock. I will be doing a side-by-side evaluation over the next two weeks. If you can afford to wait, I can give you a detailed comparison. The evaluation samples will then go on the website at a reduced price, so there will be a good opportunity to save $100 or so if you like the reticle.