A friend & I bicycled on Guernsey & Jersey a few years ago. We spent 1 1/2 days on Guernsey and just 1/2 day on Jersey. We arrived by ferry from Normandy, and returned to France by ferry to St. Malo (Brittany). We enjoyed our brief stay. My impression is that there's more to see and do on Guernsey. FWIW, our actual cycling was better on Jersey (less traffic and less road construction). The roads were quite narrow, esp. the backroads outside of the towns on both islands.
Places we visited on Guernsey and enjoyed include: the German underground hospital (there's also one on Jersey), the German Occupation Museum, the Guernsey Tapestry (a well-done modern tapestry showing the history of the island), Candie Gardens, & Castle Cornet. All were worthwhile.
Because we had just half a day on Jersey before taking an evening ferry back to France, we just biked on Jersey. We biked around much of the eastern half of the island, which was quite pretty. Overall, we could have used at least a couple of more days to see more of both islands.
There was quite a contrast between our time in France and our brief time on the Channel Islands. There was a sudden change in language, side of the road we biked on, a 1 hour time change, and the quality of the food.
