The correct answer is both.
They do sell adapters so you can plug a 50 amp rig into either 30 or 15 amp outlets and so you can plug 30 amp rigs into either 15 or 50 or a 15 amp plug into a 30 outlet.
You can also install a 50 amp outlet with 30 amp breakers if you wish to save a few bucks.. This will usually be enough for a 50 amp rig, and costs less.
you have a friend over with a 30 amp rig, See adapters above.
The only thing you should NOT DO under any conditions is hire an electrician to put in a TT-30 (30 amp) As the odds of a catastrophic mistake are very high with this outlet professionally installed.
That said I put in a TT-30 for my 50 amp rv, But **I** put it in, and I do know what I'm doing.