Is the side imaging really worth the extra cash? I can't decide which one of these to get. I'll be using it in a boat and hopefully a kayak.

The SI version has DI and 2D sonar too. 

YES it is worth the extra cost for the extra coverage it has.

 

I have four Humminbird SI units.

I rarely use my Si. More of a "toy" for me to see cool things. I can live without it. Especially on such a small screen

I got it without the SI, I wish I had the SI sometimes, however I don't think I am lost without it.  I know that sentence helps absolutely zero.

I don't have either of those units, but I think I'd still go with the SI because if you learn to use it, it'll become an invaluable tool. 

As Wayne already stated the Si will have both Di and regular sonar. I have the Si unit and really like it. Not only can I see directly under the boat but up to 90 feet (I think) on both sides of the boat. It really helped one day when the hybrids were schooling to keep up with their position. I like the Si unit so much I purchased an 899. Now the helix is at the console and the 899 is at the bow. It's another 150 to upgrade from Di to Si and you'll have both technologies, IMO it's worth it. I think it's better to have it and not need it than it is to need it and not have it.