OMG IKR!
They are the same thing, and it really isn't hard to teach new players THAC0 because yes, the chart is there on some character sheets. The thing that annoyed me most was a comment....
I still play "2E" to this day. What I discovered was that the reason why I think a lot of people had problems with THAC0 might be due to DMs not announcing what the AC of the NPCs were in combat.
Why do the players need to know? That is probably the biggest problem. Rather than worrying did you hit, just tell the DM what AC you could hit or what you rolled and he will tell you if you can roll for damage....
Sure after a while you can learn every AC in every edition, but do you really need to know? Are you going to metagame in order to use some special weapon to hit this one with tougher armor class, rather than using your best weapon all the time to try to get the most use out of it and most hits in?
Hey my THAC0 is 15 and I rolled a 14, 15-14 means I can hit anything with AC 1 and higher.

Really not that hard.
If you roll damage on it, it means you know better what AC it does have if you need it, but come on...it isn't that hard either way since they ARE the same.
It is like arguing is a zebra black with white stripes, or white with black stripes. Does it matter?
Probably watch more of them later, jsut had to see the THAC0 one first.