If you’re flying this weekend there are no tubes or trains to or from T5 so it’s easier to stay in T5 and use the lounges there.

Even when there are tubes and trains it’s not really worth the effort trekking over to T3 and back just to use a marginally better lounge.

Note if you want to use the BA arrivals lounge at T5 you’ll have to clear the UK border and go landside. But you’ll have access to any of the three Club lounges in departures in T5, which means you can stay airside.