I work as a welder and I've worn both the firehose flex, and standard firehose.
Standard firehose is thick as hell, and works well. Doesn't get frayed easily when I get snagged by sheared sheetmetal (which has a burr and is sharp as a knife), and it doesn't singe easily (well duh, firehose). It's heavy though, perfect for wintertime. Ever worn a BJJ gi top? The material and weight is very similar.
The Flex stuff is way more comfortable, a lot cooler/thinner. Same singe resistance, but it does tend to fray a lot easier. By the way those frayed edges will catch fire. Nothing like realizing your ankle suddenly feels warm only to stop welding, flipping up the hood while glancing down to realize you're on fire. I still have mine from last summer. It's obviously used, but still serviceable.
As far as sizing goes, I have pretty limited experience, and my build is apparently quite odd (since I can never find pants that fit right). The flex has more range, since... well it flexes. So my flex is 36 while my firehose is 38. My actual waist is ~34.5" but my thighs are ~25" and hips are 38.75" (see what I mean about pants that fit)?
Carhartt double front pants I can get "true to size" waist (34) and come out pretty good around the thighs and hips. Levis seem to be popular, so as another comparison: my daily Levis (the 505's) are a 36x32 and trim around the thigh & glute/hip area, and has enough waist room for a G19+light holster and 2 kydex spare mag pouches.