I have both phones. Owned the Z first and let the battery mod on 24/7. Great phone. Was curious about the Z Play and its battery, so I picked one up 3 weeks ago and have been very impressed. The battery is nothing short of amazing! Don't really see any speed or screen resolution differences.
Not an easy choice to make as they are both great phones. Depending on your use the Z should should get you through a day without the battery mod. With the battery mod you have 2 or 3 days easy, and it's not too heavy or thick with the battery mod on. And that's the real advantage of the Z. The phone is so thin that it'll make using the Moto mods much more attractive to you.
The Z Play is just amazing. And you can use the style backs and change the look of the phone all you want without sacrificing great battery.
Between the two phones the Z Play is my current daily driver. Hard to put it down.