Once you have a decently size ground force (~ 6 troopers?), you want to start upgrading them, right? Now ... if you're seeing some concrete threat like Mutas or Tanks, then you'll want a specific upgrade like Blink or Charge.

But ... assuming you haven't found any concrete intel at this point, what do you lay down first, Twilight Council or Forge?

While Forge in itself isn't that expensive, you can't seriously plan to build both Forge and Twilight Council and and have enough minerals / gas left to buy upgrades at both.

One advantage of Twilight is of course enabling High/Dark Templars. So ... how (and when) do you decide which of them to get first?


Question: Why choose an 80-200mm over an 18-200mm lens?
Answer: The biggest reason for difference in the two lenses is aperture. The 80-200mm is a constant f/2.8 throughout the focal range and the 18-200mm varies from f/3.5 to f/5.6, so substantially slower, especially at the far end. All this really means is that the 80-200 can let in more light at the same focal length over the other. 

Also, generally, zooms with constant apertures are higher grade lenses. I hesitate to make this statement a truism, but it pretty much is. Consumer grade lenses are often massive zoom ranges with variable aperture whereas more professional grade variants are smaller zoom ranges and constant apertures. The professional variants tend to be fast, sharper, and better built. There are exceptions, but this is generally the case regardless of brand.

So, that is why you might make the choice of the 80-200mm lens. I made such a choice with a Pentax variant not so long ago...

Edit

To answer your other question, I would probably recommend the super zoom (18-200) for a newcomer if you want a single, general purpose, lens. I would expect the 80-200 to be optically superior, but also more expensive, and it would probably mean a second, equally expensive, lens to fill in the range. Worth it to some of us, but not for everyone.