Since the Pi inputs are 3.3V, you probably should go with 3.3V to power the ID-20LA. At 3.3V, the ID-20LA current draw is about 18 mA both when idle and when a card is in range. At 3.3V the range is reduced to about 5 cm (using a card type RFID tag). At 5V, the current draw is about 27 mA, and the read range is 7 to 8 cm using a card tag.
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