Regardless of brand, get a telescoping model
Although we've had a Forspar telescoping extension for 5 or 6 years, I've seen other brands that would probably have worked just as well. I initially chose this brand because of the way the attachment point to the aluminum tiller was set up. It makes a nice, solid connection. When you decide, it would be a good idea to get a few of the u-shaped plastic brackets you can snap the extension into place on the tiller when not in use. Check the size you'll need, they're only a few bucks each, but seem to last me only a year or two. Also important...make sure the extension telescopes far enough so that you can use it comfortably from a stern rail seat. Good luck.