got a question about two ps4's dont know which to mod

hi my current ps4 is on version 5.05 so i guess im kinda lucky that i didnt update
its an old ps4 the standard one i ordered myself a slim console checking the firmware its on 3.55
should i keep my slim for my online play? and wait for a a release for the 5.05 for my normal ps4?
just some questions