Hi Santiago, I salute your choices! RSS and Markins make some of the best ball heads on the market and I too went through a similar process of weighing them up last year, albeit smaller versions for a Gitzo 1541 T.
Unfortunately where I am, I had no chance to try them in person nor any chance to try review samples, so I had to rely on personal recommendations on various forums. Annoyingly though, few if any of the people commenting had actually tried both types, so I couldn't find a definitive comparison.
In the end I opted for Markins because their smallest model was designed to work with the backwards-folding legs of my 1541 T legs. I've certainly been very happy with it. It's a superb piece of engineering. But from what I've heard, the RSS heads are equally good. My friend Trey Ratcliff of 'stuck in customs' certainly loves his RSS head and legs, although I've not had a play with them myself.
Maybe the choice of camera plates will influence your choice, and as I recall there were more for the RSS system (although you could mount an RSS quick release bracket to someone else's head if you really wanted). So first check if there's a quick release plate available for your camera from both companies - I'd really recommend looking into an L plate.
This might make the decision for you.
I think the bottom line though is you'll be equally happy with either head, so a lot will boil down to price, brand-loyalty and customer service experience.
PS - when you buy your Manfrotto legs, please consider going through one of our partner stores on the support us page; B&H and Adorama will ship internationally. Sadly neither RSS or Markins are sold by any of our affiliates though!