My 2 cents. I have both; purchased from SWFA. Of the two, I would purchase another 4200 3-9x40. I do not like the BDC on the Primos it is too thick and useless unless you are willing to take the time to learn how to use it. I hunt out to 200yrds, a BDC is not necessary for my application. I do not think it is as clear or as quick to bring to my eye as the 4200. I got caught up in the "it's a Monarch" hype. I mounted the 4200 on a Savage 243 and have been very pleased.
If I had it to do over again I might try a Vortex Viper or Conquest; although they both are getting a lot of press as well (maybe deserved, maybe not). I have not seen one in person. Read these sites long enough and you will find the answer you want to find to justify your purchase, I did. My usual is a Leupold 3.5-10x50 VXIII which I think is an outstanding scope but I'm a sucker for peer pressure and read too much...study long, study wrong.