Home base for Exploring Devon and Cornwall

I think we'd need to know what places you want to see in order to have an idea of whether buses would work. I suspect an examination of schedules will show that needing to make bus connections might bring the whole itinerary down like a house of cards. If you stay somewhere with single-bus access to places you want to go, it may be fine.

From my experience either Mevagissey or St. Austell works for visiting the Eden Project and the Lost Gardens of Heligan.

Staying in two different locations (2 nights / 3 nights) would give you more options.

St. Ives has rail service, which might open up some good possibilities.

It is probably best to stay somewhere central rather than miles down narrow roads in some cute coastal village - which would have little or no parking. Truro would make a good central position as a base for Cornwall - just make sure any planned accommodation has free parking. Now, I am not saying stay in Truro - just aim for that general area. Look at the map:> https://www.google.co.uk/maps/@50.3192819,-4.7971517,97112m/data=!3m1!1e3!5m1!1e1

Try and see Looe, Polperro, Fowey, Eden Project (St.Austell), Trebah Gardens, Glendurgan Garden (if time) , St.Michaels Mount (from Marazion), Mousehole, Minac Theatre at Porthcurno, St.Ives, Padstow, Tintagel. Car is best for getting around. Zoom down on the map and check out street view of some of the roads. If you want to use public transport > www.traveline.info
https://www.visitcornwall.com
https://www.farmstay.co.uk/PlacesToVisit/Westcountry/Cornwall
If you want value accommodation:>https://www.travelodge.co.uk/uk/hotels-in-cornwall

Personally, I would opt for a car over public transport. Sure, a bus will allow you to view the passing countryside above the hedgerow if it's a double decker but there are too many cons in comparison to driving. With a rental car you are not constrained by timetables and routes and when considering you may have to wait for an hour or more for the next bus if you miss one a car makes for a more attractive proposition. Bus travel is also quite expensive in the UK and several trips will soon mount up particularly if there are four of your. The costs of a car rental when split between two couples will be significantly cheaper than a number of bus trips. There is also the freedom to visit places that can't be reached by bus. Besides, not all the roads in Cornwall and Devon are flanked by impenetrable hedgerow, most are not so you will still get to enjoy some great scenery whilst driving.

As for a base, there are so many areas in that part of the country that would make a good base that it's almost impossible to recommend any particular one. My option would be to look at renting a cottage somewhere, perhaps along the Jurassic Coast. We recently rented a fantastic cottage near Lyme Regis and we were able to walk to the shore and search for fossils but also be within easy travelling distance to places within Devon, Cornwall, Dorset, Somerset etc. There are many, many cottages to rent within the area and it can take hours of searching and deliberating!

As you only have five nights (four whole days) and there are four of you, I'd offer two pieces of advice.
1. I second everything the previous poster wrote about the advantages of a rental car, especially with 4 people.
2. If you want to see Cornwall--which is lovely and fascinating--choose a base location in Cornwall and enjoy. Distances are further than they look on a map (in terms of time to get from one place to another) and you don't want to be spending hours every day cooped up in the car (or riding buses) just to be able to say you've seen both Cornwall and Devon.

Thanks for the input. You have pretty much reinforced what I was thinking:

1. If we want to see Cornwall we should have a base in Cornwall.

2. Renting a car seems to be the way to go.

Thanks!