Which RAM option would you choose?

Ideally you'd buy all the same ram modules and even more ideally, you'd buy it all at the same time.
Mixing ram can get you into trouble with speed, cas latency timing, and voltage differences among the modules.
If I were totally buggered and had to go with different RAM I'd try to stick with identical speeds for example if DDR3 is in one slot I'd look to have DDR3 in all the slots and not try any other type even if I had to use different manufacturers.

 

More RAM at the slower speed will ideally work better in most instances... You might experience issues in conflicts though depending upon the design as Raul said... Technically the system should reclock all the memory to the slower speed and all should be happy, but sadly the real world is not the ideal world and it doesn't always go so smooth and you might encounter issues... And memory issues can be hard to diagnose, they can cause so many issues it can be a headache to track down...

Ironically if you research how memory manufactures work you will see that they almost always try to make the fastest memory, they test it and if it fails they test it again at a lower speed, and again if it fails they test at yet another lower speed... When and if it passes at a certain speed that is what they label it as... This is also the difference between first rate memory and second rate memory... First rate memory like Micron for example will toss all the failed memories into a reject bin and sell it to a second rate memory company that will do the actual retesting at lower speeds... This is why most 'pros' will recommend first rate memory, as you really never know what you are getting from the second rate vendors and you don't know what specifications it was tested to...

 

you could download MEMTEST and put your individual memory chips to a speed test
That might allow you to match speeds at least so if all your chips were of the same generic type you might be able to pull it off cleanly ( might, being the operand term)

 

You can always test it out with all the memory installed and see how it works, it might work just fine like it's supposed to, or it might not...

If you have issues like locking up, crashing, blue screen, random reboots, or basically just about any other reoccurring error it might be time to remove the mix-matched modules... If you don't have issues roll with it...