For me ‘live looping’ is guitar, cable, OT and 10 button pedal board performing to an audience of myself to create new grooves & tune ideas and to jam away infinitely on a layered multi-track riff.
I was interested in the RC505 but prefer the OT as once I have either pickup machines or flex recorders set up to loop (using hold setting) then I feel like I have far more post-recording options to use my recorded audio for other purposes within the OT than if it were in the RC505. Furthermore I already have an OT and the RC505 would be yet another (bulky) piece of gear.
The main advantage of the RC insofar as I can see is that if I were performing in front of an audience it is possibly far less finnicky than the OT, with its big buttons and user interface - unless you are well practiced in using the OT and your pedal board.
All-in-all if you already have an OT, you don’t ‘need’ an RC505 to get a lot of mileage for loop studio work and subsequent chopping, editing and mangling … but for live work, the RC505 may be a more comforting & immediate experience
And as jaschar says, the RC505 is a dedicated looper, just switch on and go … dammit, you’ve re-kindled my rc505 interest!