hanging 140 lb on wall with metal studs
I have wall mounted a number of very large TVs on metal framing, some even with articulating / extending mounts.  Since they were retrofit, I just ran toggles into the steel studs.  Although with the articulating mounts you could see the wall flex a bit when the mount is extended, it was always secure.  

Using toggles in metal studs isn't too difficult, but you do have to locate the studs precisely to drill the holes for the toggles.  

Some recommend horizontal blocking between the studs to make these mounts work better, but I would not go that route.  I'd sister wood studs to the metal studs where you want to fasten to the mounting bracket.  You might want to double up the steel studs (make a box) as well while you're at it.  If you double the metal studs up, you can put wood studs either to the left, or to the right, or on both sides of the stud.  

If you were installing an articulating mount, you might want to add a stud and / or change the stud spacing to stiffen the wall and spread the load.  In fact if you think you may ever change to an articulating mount, you might want to do that now.  

Make sure you can reliably locate those studs after the fact, take measurements, notes, and pictures, and mark the subfloor where the wood studs are.  A stud finder alone won't tell you where the WOOD studs are.