I use both. I like MT5 better because of its interface. It is a bit more modern. Also the strategy tester is better in my opinion. It can do multi currency testing.
MT4 is a bit more old school.
I never noticed any performance differences.
Keep in mind that there are many more indicators and EA’s available for MT4 if you are into those things.
I use exclusively MT4, most brokers use mt4 as well, mt5 has different class of assets and functionalities tho, which is interesting, but now with the newest update of mt4 it looks even better