RAM Question: 12800 @ 8-8-8 vs 10666 @ 9-9-9

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12800/8 = 1600 MHz 8-8-8, or 10666/8 = 1333 MHz 9-9-9

Based upon above, clearly the 1600 MHz 8-8-8; faster frequency AND faster CAS {lower}.

Well it depends upon the CPU and MOBO; so what CPU and MOBO? You want to make certain that ANY RAM is compatible with both components. Both 8-8-8 AND 9-9-9 are considered 'tight' timings; whereas 8-9-8 are considered loose timings.

If the question is More Slower vs Less Faster:
4GB {2X2GB} 12800/8 = 1600 MHz 8-8-8, or 8GB {2X4GB} 10666/8 = 1333 MHz 9-9-9
... then the Answer is it DEPENDS. It depends upon what you're doing, if you're Rendering then More Slower - WINS! However, if you're Gaming/Normal Apps then Less Faster - WINS! Further, if your OS is 32-bit {4GB MAX} then Less Faster - WINS!

Speed alone 1600 MHz 8-8-8, or 10666/8 = 1333 MHz 9-9-9; the 1600 is about 2~4% faster. If you did not manually set your RAM then you've been loosing 4~8% performance depending upon what Auto picked-up from JEDEC SPD.

Thanks for the welcome and the detailed answer  

I should have provided more info indeed.
CPU: i5 760
M/B: Asus P7P55D-E
O/S: Win 7 64bit
- No overclocking done -

I'm mostly using my pc for gaming and music apps (Nuendo, Cubase and various VST instruments that are quite memory demanding).

Depends on what you are doing with your PC... if you multi-task a lot you will see more of an improvement with higher capacity; if you are talking about gaming then you want to go for lower timings.

If you running at or above ~75% Memory usage then you DO need more RAM, it not then it won't help too much and you might loose 'some' performance.

Options:
Matching Set of OCZ3G1600LV4GK $105 http://www.amazon.com/gp/offer-listing/B002MOQNKI/ref=d...

Corsair Tested -> http://www2.corsair.com/configurator/product_results.as...
Tested $99 CORSAIR Vengeance 8GB (2 x 4GB) DDR3 1600 Model CMZ8GX3M2A1600C9 {2X4GB} http://www.newegg.com/Product/Product.aspx?Item=N82E168...

G.SKILL Tested -> http://gskill.com/configurator2.php?pid=2&model=1325
Tested $99 G.SKILL Ripjaws Series 8GB (2 x 4GB) DDR3 1600 Model F3-12800CL9D-8GBRL http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Tested $85 G.SKILL Ripjaws Series 8GB (2 x 4GB) DDR3 1333 Model F3-10666CL9D-8GBRL http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Kingston Tested -> http://www.ec.kingston.com/ecom/hyperx_us/modelsinfo.as...
Tested $99 Kingston HyperX 8GB (2 x 4GB) DDR3 1600 Model KHX1600C9D3K2/8G http://www.newegg.com/Product/Product.aspx?Item=N82E168...

While the RAM you listed 'might' work, it's not guaranteed. 4GB sticks are 'picky' and I've had experience with CMP8GX3M2A1600C9 working with no problems. Anything else, I would start a Ticket with e.g. Corsair; the vanilla answer is 'it should' but I don't like guessing with compatibility to avoid issues like -> http://forum.corsair.com/v2/showthread.php?t=90944

...BTW this is why I asked about your CPU + 1600 {DOCP}/yours is 2.8GHz:

Great info, thanks.

From what I see, when going above 8GB it's not possible to find lower than 9-9-9 timings without paying almost double as much. So I suppose, I could actually keep both kits and play around with them to see if I can notice any difference.

For most people the differences do not justify the price differences.

From the list I probably would choose either the Corsair Vengeance or Ripjaws; Ripjaws if you have a CPU Fan that covers the DIMM slots.

I'll check out those solutions. Thanks for the pro info.