Macbook Pro Upgrade - 16GB RAM or 128GB SSD I am asking myself which constellation is better: should I stock up my RAM up to 16GB (from 8GB), or should I buy a SSD with 128GB?? 

I really dont know :/

Cheers!

BTW: My macbook supports 16GB RAM, i already checked this ;)
RAM upgrade above 8GB if...

you do run Virtual Machines excessively because you need to allocate a lot of RAM to the virtual OS
Other than that, this will most likely not be necessary. Mac OS X operates very efficient with 8GB of RAM. If you do not run VMs, there will  be no increase in performance in most cases!

SSD upgrade if...

you want a snappier response throughout the system.
you run big programs such as Adobe or photo editing software that take long to load.
you need faster boot/shut down/sleep times.
if you carry your MBP around a lot. SSDs are very shock resistent. No mechanical parts move.
if you want a more quiet MBP.

SSDs also consume less power. But you won't notice it significantly.
Think of this:
In today's computers, the bottle-neck of processing is usually the hard drive. Processors generally need to wait for the hard drive's read/write operations which are a lot slower. Thus a faster hard drive is needed for a better overall performance in processing. RAM won't help if the needed amount is saturated.