can people still get in and out of the backseat?

 

They can with the smaller one. The bigger one has a bar goinr right across the back of the front seats, and a bar going up diagonally that would be acroos the rear doors. You'd have to see one already in to make that call. It you want it for safety only, then just get the smaller one. i doubt you would need a bigger one unless you were into motorsport. The bigger one will stiffen up your chassis a hell of a lot, but the payoff is banging your head on bars, and having a playground swinging bar in the back seat.
I had a roll cage in my old race prepped astra, and it would have been a PITA, but luckily it was a 3 door, and only a passenger would be in at the time. You can sort of see it in these piccies.

 

You can see more clearly in this piccy...



the rear portion of the roll cage will block the doors..