2" line is a little more maneuverable, but are limited with GPM as opposed to a 2 1/2". If you are already carrying 1 3/4" line as an attack line, why have your next bigger line only a 1/4" more diameter?
I see the 2 1/2" being a bit more versatile as a supply line, an attack line, used for offensive or defensive, the bigger diameter does give you more GPM.
Welcome to the biz. You will find this forum an excellent place to ask questions you may feel shy about in your department. Read all you can and check out some of the instructional videos on YouTube.
Regarding 2 1/2" attack line, this is a beast of a line and requires a lot of manpower to advance and manoeuvre. For exterior operations, it's a good option as it's portable and easily moved. It will put out a hell of a lot of fire if used properly.
For interior operations, the situation changes. It is very difficult to move it in most buildings and can hinder your ability to attack a fire. Here's a LODD caused in part by the crew's inability to advance the attack line past the first room.
Don't underestimate the 1/4" of extra diameter. The diameter of the hose increases at the square of the radius (diameter=3.142 x Radius(squared)) so you're getting a fair bit of extra water with a 2". We used to carry 1 3/4" as our primary attack lines, a 2" with a smooth bore for the bigger stuff, 2 1/2" for exterior ops.
FWIW - a well deployed, well used hose line is more effective than a massive one poorly used. Here's a video of a massive housefire extinguished with 1 1/2" lines.
I agree with your comments about the 2 inch hose, and technique trumping tools.....thats why the 2 1/2 can be stretched interior with 2 to 3 guys with the right technique.
As for the PG video....I think this proves the need for the 2 1/2 that 1 3/4 in my opinion did not put that fire out. It did not have the GPMS to do so. The fuel simply burned away enough over time in order to make it an 1 3/4 fire. Besides the fact that they accomoplished nothing, saved nothing. The 2 1/2 (properly tipped witha smoothbore nozzle) would have been able to hit the fire from a safe distance and provide 300+gpms. The 1 3/4 on the fire in question was a completely self serving (do what I wanna do not whats right) and "Teddy Bear Tactic" (mad them feel real good but in reality had no bearing on the outcome of the fire)
correction - I use the word "diameter" where I should say "cross section." Meh, you get my drift, for a small increase in diameter, you get a large increase in available waterway, and an even larger reduction in friction loss.