You can always walk back on to the island (except during some extra high tides, I think mostly in September, that cover part of the bridge).
The important question on whether to stay on or off island is budget related. Sticker shock chased me off the island. There are, I believe, two hotels on the island. If budget is less of a concern than energy expenditure (which it is for many), staying on MSM is probably wonderful, you will be right there after the crowds have dwindled - which is certainly valuable.
If you'd rather save significant money even if you have to walk an extra couple miles (it's pretty far from parking to the island and back), stay off island. There are plenty of hotels in the area. I ended up staying at a hostel toward Cancale.
Food was an even bigger sticker shock. The traditional omelettes looked great so I planned on getting one on MSM for dinner. Unfortunately, the price for a single omelette was 35-50 euros (Sep 2016), nothing extra included (not an exaggeration). I ended up having oysters and beer in a Cancale cafe overlooking a small bay for less than 20 euro - a typical cost for a meal (outside Paris) and arguably the best oysters on the planet.