If you want to do browsing + phone calls + whatsapp, the Z3 is more than worth it.

THe only cons are the camera (which is a 5mp unit), and the 1.5GB RAM (which still makes it a good machine, I've got a 2GB Android, and I'm always only having 100-200mb free, dafuq?)

Depends on what you want to compare.

If its camera, the Zenfone 5 (it has 8mp, and that actually-usable-low-light-camera) can shoot better.
Screen size should be the same.

Power? The Z3 win hands down. Ridiculous power management from the Zenfone 5, despite zenfone 5 having a slightly bigger battery.

USB OTG is only very recently (like less than a week) introduced to Zenfone 5. BlackBerry Z3 still doesn't have it (which  is kind of moot, if you're only using the USB-OTG to read thumbdrives as Zenfone 5 and and BlackBerry Z3 actually can and will do hotswap of SD card, but eventually depend on the use case, since BlackBerry Z3 will have "BlackBerry Blend".

Android apps, as of 10.3.0.1052, works well, again, except if you need GAPPS.

In the end it depends on what you want. Do you want a BlackBerry, or do you want a full fledged Android experience? I've got both and I want a BlackBerry.

should be possible, I sync my google contacts to my Q10