I've handled a couple of their scopes. At the competitions I've been to in the US, Sig has been very active with sponsorship and demos. At Granite Creek the winner of the Tactical division was using a Tango 6 and was in the same squad we were. Got to spend some time behind it and really like the Level Plex led's in the tango 6. They are decent and have a good choice in the Mrad Dev-L reticle. Decent enough that the guy I shoot with is looking to sell his PM11 5-25 PLF4 to get the Tango 6.
The Tango 4 looked to have the same glass quality but did not have the Level Plex system, limited to 4X zoom and is only available with a 30mm tube with 50mm objective. They do have the Mrad Dev-L in FFP
Myself, I'll stick with S&B for now on my main rifle but will look to get a 3-18 for my LPR SR-15.
It would be nice if we could get access to them here in Canada. Seems to be by order only and that might take a while. Unless someone has seen stock recently they are unobtainium... Had lots in the US when I was there. Tango 4 6-24 with Dev-L $1100. usd. Tango 6 5-30 with Dev-L $2600usd.
Way more scope on the Tango 6 for way more cash... Tango 4? I'd go with Burris or Vortex.