I ride a Z1 for a year now, and have a few comments:
The only reason for the abundance of "sloped" top tubes nowadays is that it enables mass manufacturers to reduce the number of different frame sizes required to provide an acceptable fit for most riders, for pure cost reasons (it was invented by Giant in Taiwan back then). Before this, sloped top tubes = girl's bikes. This became especially an issue when production moved to (monocoque) carbon fiber, where each frame size needs a separate, expensive mold.
Now, on the other hand, the only reason to go for a Z1 over a Z2/Z3 (which otherwise are on the same level as the Z1), or -to that end - many other fine bikes from other manufacturers, is the ability to get a fully custom sized frame, which eliminates the need to use shortcuts like sloped frames.
That said, the lack of a sloped top tube on size 54 or 55 seems a bit out of date.
It is only out of date because of said mass manufacturing cost-cutting fashion. IMHO, the primary reason for a Z1 is to evade mass manufacturing fashion. So, in my very personal view, a sloped Z1 is kind of missing the purpose. Personally, I bought it because it has a proper men's bike top tube slope angle (= 0°)
- but hey, I am also preferring external cable routing (and I am not that old).
That said: each to his own - if you really like the look of sloped frames, go for it, the frame rides fantastic and is rock solid. I think EPS is a also a good choice, as by the time you want to buy a new frame, everything will likely be electric and you should therefore better plan for e-wiring now.