It depends what you use it for. I have 16GB in my MBP, and it comes in handy with VMWare Fusion and similar programs. If you don't have an SSD, then you'll notice it even more with just regular browsing and usage as the system alone uses nearly 4GB these days.

16GB will work just fine.

 

My questions: How much better will it run on 16G compared to 8G? Is it worth the extra $100?

How much better it will be depends how you use your machine.

In short, having Enough RAM is essential (as everybody who installed Yosemite and El Capitan on machines with 4 GB RAM found out quickly).

Once you have enough RAM for your OS and its associated processes, then more RAM helps most when you are:

1) Multitasking. When you have multiple programs open at once (including background programs), the OSX is forced to write and read from the swap files on the disk when you exceed the amount of physical RAM. This is why you see beach-balling when switching between programs' windows. More RAM raises the bar on how many programs that you can multitask without delay.

2) Using programs that aggressively use RAM internally, the most common examples are Illustrator, Photoshop and other image programs, Video production and Audio production. For programs that are coded to use lots of RAM when available, the speed up can be substantial vs. using scratch files. This also depends on how big the data sets (resolution, size, number of layers) that you are working on.
Some programs allow you to allocate RAM settings internally to optimize performance.

 

Thanks for your replies. I received some constructive information from CanadaRAM and ordered 16G from them (great service, BTW). While in the process if installing them I did a little test to record the relative start up times with various RAM installed. The following is the time from RESTART to log-in window:

4G RAM 2:01 minutes
8G RAM 1:12 minutes
16G RAM 1:09 minutes

Its not very scientific but its a bit of an indicator. What I do know is that now with 16G my computer is crisp and clean again.

 

Good choice and I'd sure agree with your CanadaRAM recommendation.

Those are interesting varying Restart times, and probably don't have much in relation with your normal performance, but don't be too surprised if you redo the tests from a cold boot and find it takes longer with the more RAM, as it takes a bit longer to check and I'm not sure that a Restart does such a through test.

Your Activity Monitor will be your best guide, if not your own observation of better performance, and I'd say your choice to go with the 16GB modules was the correct and best one.