It really depends on what you are doing
You have a bunch of data to store on your computer, want to be able to run programs fast, but aren't particularly concerned about the speed at which they open (or the speed at which the computer turns on.) You also tend to run many programs at once.
go with the 16 gb of ram. This will ensure top notch system performance in terms of program speed. You also have 500 gb of storage instead of 128.
You want your computer to turn on super fast, don't have that many files to store, but often use programs that are complex and use multiple files at once that you want to work fast.
Go with the SSD. Since you are more concerned with hdd access times, this is definitely the way to go. 8gb of ram is still top of the line for modern computers.
personally, if given the choice, I would choose the 16 gb of ram. This is because I use many programs at once and really need the extra space a spinning drive provides.
I have a computer with a 1 TB 5400 rmp drive and I have rarely noticed computer lag due to the hard drive. When I do it is usually because I am copying files and running a virus scan at the same time.
In case you ever want the temporary performance of an ssd, you can use SoftPerfect Ram Disk to create a ram drive with your extra ram.
Hope this helps!