We hope to rent a house next winter in either in and around St. Augustine or Cape Coral.
My understanding is that the weather will be nicer in CC but not much to do. And the opposite for St. A.
Any pointers appreciated.

You've pretty well summed it up. Considerably warmer in Cape Coral. It's not far for day trips from there to Sanibel, Ft. Myers Beach, Gasparilla Island, Naples, the Everglades, etc. So there are things to do nearby.
But Cape Coral itself is pretty lacking in anything except industrial services and cheap strip malls.

I'm curious though how you came up with those two options? There are houses for rent all over Florida. Why Cape Coral?

If you are thinking of renting a house or condo in St. Augustine, try to do it before May. Many people from the Northeast, spend the winter months here and generally leave around Easter. They then book the next year's reservations at that time. 3-month rentals are common.

My immediate answer is - trying somewhere new. We've been to Siesta Key, and St. Pete Beach. I hear so many nice things about St. Augustine - but then it's not as warm.
We went to Cape Coral, and the houses and canals were very nice, but you're right about the strip malls, etc. We'd be driving all the time to go somewhere else.
We have dogs, so it's difficult to rent...

Cape Coral if you like to be outside in the winter time. With Sanibel and Captiva nearby and 55 golf courses, fishing etc it is the place to be. I purchased a vacation home to lease out before I retire one of these years and I traveled all over Florida. Cape Coral is tops.

How about good restaurants in Cape Coral?