c) it runs on the tried and true CS engine.
Huh? I'm thinking you're talking about Counterstrike. If thats the case then you mean the HL or Half-Life engine. Bearing in mind that HL used a heavily modified version of an Id engine.
TFC was well established before CS became the defacto online mod. Last time I checked CS had quite a few more players than TFC.
I really prefer TFC and just haven't been able to get into Counter Strike. The players on public servers don't communicate and the cheating seems so pervasive it's not worth loading up.
--What the hell, go and put all your eggs in one basket