Having owned an sony xperia phone in the past, there's no way I'm ever going down that road again, no matter what the z3 might bring to the table. Android updates take forever, there's very simply very little or no support for future upgrades, you'll probably get one upgrade to lollipop and then you'll be stuck there forever.
Unless there's been a major change in how sony supports their phones, I'd stay far away from them. I'm not a huge fan of the touchwiz bloat, but I'll give sammy credit for upgrading phones like the S3 to kitkat etc, they seem to support phones with android updates faster than sony.