well,as you said,performance wise if not a thing to you,why bother upgrade? since u dont appreciate change in pixel density and overall speed,or even camera.

then only thing u wanna ask is ui,hence miui vs touchwiz,
thats is subjective,some ppl like miui(u use s3 for 2 years,u must know miui by now),some ppl hate it,with no chance of flashing 3rd party rom,you cant escape from miui,but thats not a bad thing.

everything is smooth and speedy than touchwiz(ram consume is minimal),miui is very lightweight(400+for firmware) but functional and got tons of customization without root(everything can be change with just theme,like old style phone )

only draw back to miui is not as many feature as touchwiz(smart rotation,stay,svoice,multiwindow),but thats a good thing,in my 3 years of using samsung(ace,s2,briefly s3),those feature are cool but practically useless in my personal usage,thats why i flash other rom into it

multimedia wise not that different,its not like touchwiz videoplayer can offer any better(end up with mx player anyway), music wise,the sound is good,mi3 using same soundchip with iphones(try google),the music player is simple but functional.

service center wise,not many ppl can answer you(some of the forummer did),but i guess its not gonna be huge different than other brand sc,but rest assure,xiaomi is here to stay,they know how big potential malaysia is(according to president of xiaomi and hugo barra)

well,just throw ur s3 away,samsung done playing with you,they dint even bother put kitkat on ur device(but did on 512ram device),and imho,s3 is the ugliest phone there if i have to pick one.