Just heard about this forum and was asked to respond. I'm a guide in Grand Cayman, and would love to answer any questions you have.
First, the bonefishing is good all year round, but the weather can make a difference in fishability. Winter northers come hard and fast and keep the trade winds blowing to 20 knots most of the time. While there is usually one side of the island or other to find a lee, it is still not easy to cast in such a breeze. I'd not recommend this time of year for folks new to bonefishing... there's enough to worry about the first (or second) time around without dealing with the elements as well.
Having said that, winter brings the biggest bonefish onto the flats. Recently we've seen 7-9 pounders tailing in less than a foot of water. These big fish like the cooler water temps, and even the coldest cold front only serves to entice them into shallow water (fishing is not shut down like the Keys or Northern Bahamas).
As for which island, Grand Cayman has bigger fish and more flats, but Little Cayman has more accessible schools of smaller fish. For numbers I'd recommend Little Cayman.
Finally, if you are planning a pure fishing trip I'd recommend a fly fishing lodge somewhere in the Bahamas. It might cost a little more, but you'll almost certainly catch more fish. However, if you're interested in a little guiding and some Do-It-Yourself, the Caymans are probably a better bet. The accomodations, food, and amenities are much better here than in the out islands of the Bahamas, and the flats are easy to get to by rental car. (I say that having fished in Eleuthera and Great Exuma.) I have a lot of clients who hire me for a day or so and then go it alone the rest of their stay. No problem with me.
Finally, feel free to check out my website www.fish-bones.com
for general Cayman info and the client Gallery. If you're new to bonefishing, there is also a few tips on the Fishing page. You can also contact me directly through this site if you have any other questions.
Hope this was helpful and good luck with your planning.