I have 45 alu panniers and use them if I am going for long distance trips.
I have Ortlieb bag and I take my topcase and Ortlieb bag if I don't put panniers on.
Personally I would choose 45 L again as you don't have to load them all and it is better to have large ones to not regret when you need them large !
When you have panniers, you keep the center of wight down. With Ortlieb bag, topcase, thing attached on panniers takes the center higher.
"where the traveller goes, nobody knows ! "