I used to play TFC! I bought it with the Half-Life anthology years ago. Steam was very glitchy back then. I used to play it all the time years ago. Always played Sniper or Heavy. I usually played on the 24/7 Dustbowl servers. I don't have it installed on this computer. I've played TF2 and I prefer TFC. TFC has a much higher learning curve, if you are new to TFC you will get destroyed repeatedly, it's not easy to begin at all, whereas I found with the second game, the learning curve was toned down seriously, presumably for new players. My brother prefers the second, but I much prefer the first, it's not as balanced and requires more skill in my opinion. I think of TF2 as sort of a broken version of TFC. In the original every class had special grenades only available to that class, in addition being a spy in the older game took much more skill, you could only morph, no invisibility. TF2 is still a good game, but I'll always prefer TFC due to the skill required to play.
Sniper - Pretty much the same in both games.
Soldier - Again, pretty much the same, maybe slightly slower in TF2.
Heavy - Much slower in TF2.
Spy - Much harder to play in TFC, far less abilities.
Pyro - Much harder to play in TFC, was known as a joke class in TFC, very overpowered in TF2 (in my opinion).
Engineer - Pretty much the same, except used to have these cool grenades.
Demo - Hard to play in TFC, can't really speak for TF2.
Scout - A joke in TFC, overpowered shotgun in TF2 and is much faster in the second game.
Medic - Used to be able to infect a virus onto the other team, far harder to heal as medic in the original as you had to get very close to player, unlike with the beam in
the new one.
TF2 is a great game and won't punish you for being new. TFC is more unbalanced, but a great game if you don't play Pyro or Scout. Team Fortress Classic forever!
P.S - Sorry for the long post, I just loved this game. Best online multiplayer FPS I've ever played.