It looks like both protect the XBee from being over-driven from 5V logic. Depending on what is on the 5V side, it looks like neither meet minimum drive levels. The minimum guaranteed logic 1 output from the XBee is 0.82*Vcc or 2.7V.
The minimum logic 1 input for the Atmega168/328 is 0.6*Vcc.
If you operate the Arduino at 4.3 V, it will run at 16 MHz and the XBee will meet its Vih specs. 