Which reflectance sensor array for a line following robot?
The main difference between the two models, as stated in the PDF you provided, is the output they provide to the MCU. 

The QTR-8A will provide an Analog output, requiring a corresponding analog pin on the microcontroller. The QTR-8RC will provide a Digital output, but will allow the same accuracy of the QTA-8A by measuring the discharge time of the capacitor (HIGH to LOW time), with the advantage of higher accuracy due to the use of RC circuit instead of a voltage divider.  

Both seems pretty straight forward, but for your needs, the QTR-8RC might seem more compatible, considering the Arduino Uno has 6 analog I/O pins (you didn't mention which Arduino is going to be used, the Arduino Nano has 8 and the Arduino Mega has 16), and that the QTR sensor provides 8 outputs. The way you could accomplish that with QTR-8RC and 8 digital I/O pins is described in the PDF:


  
  Turn on IR LEDs (optional)
  Set the I/O line to an output and drive it high
  Allow at least 10 us for the 10 nF capacitor to charge
  Make the I/O line an input (high impedance)
  Measure the time for the capacitor to discharge by waiting for the I/O line to go low
  Turn off IR LEDs (optional)