I have successfully done both, and the Beetle is slightly simpler as it is smaller and has fewer pillars! Having said that, I have made my own headlinings, and fitted ready-made ones in a Bay, and it is not really that difficult if tackled methodically.
All the relevant glass must come out; if you are not prepared to do this then pass it on to a pro. I peg or tape the headlining front and rear and do the cab doors first. Don't pull it too tight or you will build up problems as you work rearwards. If it is vinyl then a heat gun used gently will help work out any wrinkles since it softens and shrinks back slightly on cooling. Overdo it and those little perforations will open up into gaping holes! I then work from front-to-back, side-to-side, glueing into the window apertures - if you use spray glue make sure it is vinyl-compatible; few are.
Glue the front and rear last and, above all, take your time and do not try to make it drum-tight. It should hang there, relaxed, with only just enough tension to pull out the wrinkles. Allow a weekend for two people if it is your first time.
Having said all this, I did pass on a Split to my ex-Bentley upholsterer to fit a ÂŁ35.00 per metre pure wool headlining... I know my limitations!
Have fun. Carl.