I only started to spar maybe a month before mpx was released and I was having a terrible time, I think my stats were like 200-400 and I had only recently figured out how to win consistently. I think the main issue I had is I only have an fmg9 and a pbp shotgun on KarrenFoSho. Don't get me wrong, fmg9 is a great gun but you need a handful of guns in complex to fight different guns. Certain guns are good counter guns and you can have an advantage in certain situations with the correct knowledge of the guns. This is why, imo, Complex is harder and rewards are a bit better.
I think MPX is great for beginners which is what I think that Anero meant about understanding the concept of MPX. In mpx you can get a feel for the style of gun you like to use and then when your good at sparring you would invest in a gun that matches the stats you like to go spar in Complex. I like to keep an open mind and think that other things will be added to mpx but again if you prefer Complex rewards then go for it
I didn't enjoy Complex spar. I enjoyed MPX and I think I got decent at sparring there, so it makes me want to invest in a good gun for Complex. That is my own experience. I think this is the experience that staff envisioned.