I see you already got a few good responses here, but i thought you might want another former Samsung-users input.
1) I have always like the UI of Samsung. Currently using a Samsung phone with apex launcher and it is something i am very comfortable with.
Question is: How much close to that UI can i pull a Sony Xperia phone?
If you use Apex Launcher, or any 3rd-party launcher, your experience will be VERY similar to that of a Samsung phone with the same launcher.
I myself prefer Themer by MyColorScreen
2) Can the night photography get better via future OTA updates?
Something like a patch/f.w revision?
I sure hope it will, though i prefer slightly darker photos with low ISO noise, but that is all about personal preference. Manual settings can sort it out to some degree.
3) I like to install good quality case on my phones. Havent seen anything for the z1 till now.
Any news on this dept? I hope the device gets good support from manufacturers like Spigen etc..
Z1C is still considered a fairly new device, and where i live people arewaiting in line to get their phone, so as case and accesorymanufacturers see that the device is selling heaps, they will start shipping compatible accessories. I got a dirt cheap wallet-leather folding case-thingy, that i might swap out since I'd like to use the magnetic dock at some point in time.
Bottom line: I have no regrets moving from my S3, and the screensize is perfect. With my S3 i even missed my S1 on some occations because of the one-hand-operating-friendlyness of the latter.
Hope this helps
Samsung Galaxy S i9000 | CM9
Samsung Galaxy S i9300 Pebble Blue soon w/red glass front and white back| Omega
Sony Z1 Compact D5503 Black - Stock for now
Samsung Galaxy Tab 10.1 Wifi 32GB | Task650's AOKP