atalanta's comments could just as easily apply to both Devon and Cornwall, or to pretty much any other rural part of the country. If you want guaranteed weather, go abroad !
Both counties have their merits. I had lots of holidays on Dartmoor as a child and a trip to Plymouth was always the highlight. There is a marvelous aquarium there and that view from the Hoe is worth travelling to see alone.
Lots of shops in a nicely laid-out town centre.
Cornwall has the edge when it comes to coastal scenery in my opinion. Falmouth would be a good place to base yourself. Lots of boat trips and lots of small, independent shops.
You haven't said what transport you will be using, but a car is essential for both counties.
I personally like Penzance...a small town with lots of character. St Ives is wonderful as well and is probably worth travelling to by train, just for the journey itself !
Good sea food available everywhere.
Let us know how you get on !