I think BuildCraft and Thermal Expansion is targeting two different group of players. The way I see it, BuildCraft requires more engineering. The transportation pipes can drop items unless your are careful and balance your supply with demand. The power system will in general also need more care if the player want to avoid power loss. BuildCraft puts more responsibility on the player while Thermal Expansion comes with more built in "safety" mechanisms.
"Conduits are better because they are smarter". This is true for some players, but for other players, like me, it means that the mod has taken away important game play elements.
Both mods will live on for a long time. If Thermal Expansion ever drops its MJ support it is probably not because BC "has died", but because there is enough support for RF around for TE to stand alone. KingLemming has already talked about giving TE some sort of world resource gathering machine. Maybe that's what he is waiting for.