I'd say the eras all coincide. For football, you can find Topps, Fleer. Score and Pro Set 1990-91 everywhere for cheap, I picked up a box of 91 Pro Set Series 2 a month ago at a local card shop, for only $15, and it was pretty much the complete 400-card set minus maybe five cards, which I got for 18 cents each on Sportlots. There was so much of it being printed at that time.
Like baseball, I'd say that the "junk" era for football started in 1989 with Pro Set, Fleer and Score joining the fray and then dried up in 1994, after Pro Set called it quits. It's not as easy to find sets from that era on, and when you do they're not as cheap.