Depends on the trip. I switched from suit-inside to suit-outside recently when I realised that on trips where I stand briefly in above-welly water I can keep my feet dry for longer periods of time, especially with a snoopy loop round the suit legs. I spent a few months with one leg in and one leg out to see which leg I was most comfortable with and generally speaking I preferred suit-outside. It means I can bottom P8 in fairly wet weather and keep my feet dry for a digging session. I was able to keep my feet dry to the far sump of Notts2 recently, in wet weather, until I went for a plunge in the sump pool and then floated back home along the streamway.
If I know I'm going to be very submerged, like the Mucky Ducks in Peak, I put my wellies on the outside so I can empty them easily.