If you had to pick between...

It has been several decades since I was in either. If I only had to pick one to stay longer it would be Cornwall. I liked visiting the different towns in the area and enjoyed the shops and people of Cornwall. The Lake District is very scenic, but I found it a little less interesting other than the scenery. Neither are bad, but I feel they are entirely different. I guess it really comes down to what your expected activities are to be while you are in either area.

Perhaps those who live in the UK can better advise on this choice. Enjoy your choice.

I would choose Cornwall, there's more variety, more things of interest, different landscapes, environments etc. Whilst the Lake District is spectacular its main draw is hill walking, Cornwall simply offers much more.

They are not similar. Choose Cornwall

I agree with choosing Cornwall for a longer stay. I have visited both and while I did enjoy the Lake District, I really loved spending time in Cornwall for all the above reasons. Whichever destination you choose, I am sure you will have a wonderful time!

Both places are very scenic, however, I think the Lake District is best for hikers. Not sure about fishing, since we don't fish and didn't ask about that.

Cornwall has more history and still plenty of scenery.

If you want to walk, head to the Lake District and if you want more towns, pick Cornwall. I prefer Devon and Dorset to Cornwall.

I wouldn’t stay in Keswick, as I prefer the western LD - Eskdale would be my choice.

As others have said, there is a greater variety of things to do/see in Cornwall. If you are in the Lake District during a very rainy period, it will significantly detract from your experience. In Cornwall, on rainy days you could visit towns, stately homes, or museums like the Telegraph Museum in Porthcurno.

If the Lakes I wouldn't recommend Keswick as a base. Definately out west, Eskdale or Wasdale.