<< how easy it is to install? >>
Piece of cake if the substrate is decent.
<< Armstrong claims that their tiles fit pretty snuggly with each other >>
That's a fact. They fit snugly by snuggling close together.
<< I am kinda worried about the seams I am going to see >>
Don't obsess over it. Choose a pattern that minimizes seams and you'll be
surprised at how soon they become invisible.
<< Is the vinyl sheeting easier to install than the tiles? >>
Yes, for a professional or highly skilled and experienced amateur. Don't even
think about it if your rooms aren't perfect rectangles with no obstructions or
trim to fuss over.
Final observation, buy the top of the line tile with a polyurethane wear layer,
non wax finish or whatever they call it. FWIW, I've got that type of Armstrong
tile in a high traffic area that has stood up to a half dozen years of abuse
and looks great. HTH