It's quieter and many offer a swimming pool which is rather unusual for Venice hotels. They will offer garden areas which come at a premium on the main Venetian island. The Belmond Cipriani not only features a large pool, they also have tennis courts and you'll see small animals walking about.
You will have to take a boat to the main island and back to your hotel. These are usually complimentary and take from less than 10 minutes to about 20 minutes. I just loved sailing along in "my" vaporetto (private water taxi) as I was whisked across the canal from the Cip (Cipriani) back to Venice proper - it was a small motorboat which holds 1-8 passengers so it's quite luxurious. However, if you stay at the newer JW Marriott resort, its island is about 20 minutes from Venice so it can add a certain amount of "travel" time going back and forth. And, their boat(s) while very sleek, are much larger than the small Cipriani vaporetto. Then, you will have to co-ordinate your schedule with the times the shuttles run and after a certain time a night they are shut down and you would have to arrange a private vaporetto to take you back to "your" island. Finally, keep in mind that you won't have as extensive a variety of evening entertainment options as you would on the main island.
As for me, I think I'll alternate - one visit on the main island, the next on a private island, and so forth.
Italian Ventures travel agency in Fresno at Piazza del Fiore, Kingsburg, & Visalia, CA