I have been to the Channel Islands many times although my last visit must be about 7 years ago. Hire a car from airport or near to it that way you will see more and at your own convenience
I prefer Guernsey to Jersey reason being, Herm and Sark are a lot closer to Guernsey than Jersey, and they are a must to visit. Also I found Guernsey prices to be a lot cheaper than Jersey not sure why that is maybe because Jersey is the main Island
Make sure you try the local lobster
In all the Channel Isles is beautiful and offer decent weather and most of the locals are friendly and speak English.