[Xperia Z5 Q&A, Help & Troubleshooting] LitoNi:
I have used the Z3 for a year and currently using the Z5 for over a month, here are my few cents (both devices I used are completely stock, they are not rooted or BL unlocked):

Appearance

No doubt the Z5 looks stunning and much better than Z3. Z5 extremely feels much better to hold than Z3 because of the flatter sides which reminds me of the original Z. On the Z5, there is a black frame on the front which I thought it was going to be a hassle to hold but I was wrong. The phone feels great in hands. One thing I extremely dislike is that they removed the charging dock on the Z5, instead they replaced it with the flapless USB port which is cool but not convenient. I wash my Z3 often but I can quickly wipe the phone dry and charge it with the docking port. However, on the Z5, I have to wait till the USB port is dry before I can charge which is annoying because it may take up to an hour for the port to dry. I do prefer the placement of the volume rocker on the Z3, its not much of a big deal on the Z5 though but may take some time to get used to.

Finger print reader/power button

I don’t use the power button often like little to none on both devices. The finger print reader is somewhat a hit or miss depending if my finger is wet or dry. Dry of course is much easier, but when my finger is sweaty it may not work after 5 tries. It’s a bonus to have the finger print reader but I am not extremely “wow” by it on the Z5. Its fast though just not as accurate as I hope it would, haven’t tried wet fingers on the iPhone 6S or Samsung S6 though so cant complain here. Sony also went with a flatter power button/finger print reader which sometimes i find myself not scanning my finger on the right place. The sides are flat and the reader is also flat, so if you're not looking you might miss it. Not a big deal though as the power button is slightly dented in.

Battery Performance

The 200 mAh difference makes a huge battery performance difference. I used to get 1~1.5 days on my Z3 whereas on the Z5 I have found myself repeatedly charging throughout the day. This depends on my usage, on light usage I may require to charge once during the half the day for a short period of time, but on heavy I might charge twice or once for a longer period of time. Keep in mind I do not charge my devices over night. I usually keep it at 100% before I unplug and go to sleep. I have purchased Quick Chargers 2.0 for home and car usage, so I can easily get throughout the day without a sweat. And NO quick charge 2.0 does not work on Z3. I think it only works on Japanese Version of Z3. If you do not own a quick charger and then getting throughout one day would be a problem on the Z5. And believe me, Quick charge 2.0 makes a difference, I hated how I had to wait to charge on the Z3 because it will take forever. Rumours say Android 6.0 will make the battery performance much better on the Z5, who knows….

CPU Heat

The rumours regarding the S810 chip is true. I do find the Z5 heating up faster than my Z3 under heavy and light usage. Now, Sony may have added two heat pipes into the Z5 so the back isn’t as “hot” as some people have described. To me, it is “warm” sitting around 42 degrees Celsius (max). Keep in mind that I live in Vancouver, the weather here is much cooler at the moment. Don’t know about summer, will have to wait till next year.

Display

The screens on both devices aren’t superior but decent for the human eyes. The Z3 has a cooler colour (blue-ish tone), whereas the Z5 is much warmer (reddish tone). But it doesn’t matter as much as you can adjust the screen colour in the settings. Both devices aren’t as bright under the sun. I haven’t encounter a device that is bright enough lol. Aside from the colours both screens pretty much looks identical.

Performance

Both devices perform extremely well under almost any circumstances. It is buttery smooth throughout the whole OS. I had fully encrypted both devices, including the SD card. The difference is the 4 (32 bit) vs 8 cores (64 bit). I find the Z3 load up apps a bit faster than the Z5 due to the higher clock speeds. But the Z5 performs better on multitasking when switching between apps due to the cores. The only issue I have is the “clear all button” on the Z5 recent task menu, it lags there even on the newest update. Not a big issue but I hope they fix it.

Camera

I am not a professional photographer so I cannot comment much on this. But base on my average use as an average consumer, I do find the Z5 produce slightly better images than the Z3. I currently have a defected camera on the Z5 so I am not going to go into details.

Audio

The audio quality on the Z3 is better than the Z5. I say this because I use the speakers a lot. On both devices I find the top speaker to be slightly louder than the bottom. The Z3 produce a much deeper and clearer sound (more bass?) in compared to Z5. In terms of loudness, they are roughly the same.

Overall, there are pros and cons on both devices and it really depends if you want to the new look and finger print scanner on the Z5. Or whether you have the money to burn. The Z5 is definitely not a “wow” when jumping from a Z3 or Z4/Z3+. I do not regret getting the Z5 because i extremely fell in love with its appearance.