Here is the scoop on how 16:9 and 4:3 will display.
16:9 will look good on your wide screen TV. It will be slightly "squished" on a 4:3. The good thing is that, given time,everybody will eventually have the 16:9 format.
Even on a 16:9 your 35mm scans will not compltely fill the screen because the program scales the picture to get it to fit completely on the screen and a 4x6 or 35mm slide does not fit perfectly on a 16:9 format. Plus it scales to make sure none of the picture is in the "overscan" part of the screen. Still, with this format, you will get the biggest display.
With the 4:3 format show, you will get a show that displays correctly on a "regular" TV screen but even here the picture will not fit perfectly because 4x6 does not fit into 4:3.
The good thing about the 4:3 format is that you can change the way things are viewed on your widescreen TV so that you can watch the show at a normal size without being stretched.
Personally, I would go with the 16:9 format, just because it will have the most compatability in the future and will display fine for you now.