I'm about to buy more AK74 mags, but I'm being indecisive. I have 5 orange E. German bakelite mags and can buy plenty more for as little as $5 each. But, I kind of like the way the Bulgarian black synthetic mags look, however they're twice as much! My AK74 has black K-Var furniture, but that might change.

So, is there an advantage of one type of mag over the other (ie. strenth, reliability, etc.) or is it purely cosmetic?

There may be an extreme test, but I have never heard of either failing for any reason. For about the same price as black poly, the chocolate/olive Bulgys at kvar look about right with either wood or black plastic.

The brown Bulgarian synthetic mags at K-Var are nice.  I just ordered a couple & the pix on their website don't do justice to them.  Sort of a Hershey chocolate brown.  Might look weird in a AK with black furniture.....but not as much as the funky orange/red bakelite mags.  

I will say the bakelite seems a little heavier & "tougher" than the polymer mags but the polymer mags aren't as slick (rougher texture + ribs on front & back) as the bakelite...something to consider with wet/greasy condtions.  And the bakelites aren't exactly "tactical" being rather loud, polished red/orange with shiny black painted floorplates vs the satin brown or black mag body & satin black parkerized <?>floorplate of the polymer.

Quoted: in the time honored tradition of ar15com...GET THEM ALL...LOTS OF THEM ALL!!! they all run fine...what's not to love about them all!! more mags! just a few more mags and i'll be fine...honest!

 When I first got my 5.45 I figured the only mags I'd ever need would be the E German bakelites.......now I'm considering ordering more of the chocolate Bulgarian, a few of the black Bulgy mags, a Polish steel or 2,  & maybe a Russki bakelite for collector purposes.  Perhaps a 45rnd RPK mag or 2 if the  $$$$ is reasonable (real deal RPK74 mags, not the Joking frankenmag 40rnd)    
I'm even considering buying some of the Hungarian 20rnd 'tanker' 7.62x39 mags & I don't even have a 7.62 Ak (yet!!!).

I'm not much help here.  I have subscribed to the "more is better" theory.  I get more E. German bakelites for the same dough ponied up for the synthetics.

However, I would grab some "fancy" mags in a heartbeat if the price was right.

The brown Bulgarians look pretty good with black, IMO.  

This is a nice illustration of the effect:  

Quoted: The brown Bulgarians look pretty good with black, IMO.   This is a nice illustration of the effect:   www.aa-ok.com/11122x.jpg

Man, that does look sweet.