I wouldn't probably even buy a real stringed bass that played Rock Band 3 PRO mode ala the Squier if there was another option. Not saying the Squier sucks or anything, but I am very happy with the Mustang for PRO guitar mode.
I would be equally (actually even happier) with a bass equivalent with buttons and everything.
I'm not looking for an electronic bass instructor. I just want to play a bass like instrument in the game.
A bass peripheral like the Mustang is about 100 times more likely to materialize for Rock Band than another Squier equivalent.
So, like I said I want a game controller not an instrument. I play bass, I have basses I don't necessarily want to play my own bass in the game if it won't detect where my fingers are (I just happen to like that about the Mustang). Also I want my instant feedback when I make mistakes. I want my streaks tracked and just about everything else that Rock Band 3 will do that Rocksmith won't.
I want to choose my difficulty level and stick with it. I don't want "lives" that I lose that force me to restart after 5 mistakes etc.
So emphatically if it's a choice between playing a real bass in Rocksmith or sicking with playing the Mustang to simulate bass (for lack of a dedicated Mustangesque bass controller) then I will chose the latter every time.
BTW, GNF I can't believe you have to guess at my gender.