I would use the 7.5V input, or a 6.5V input if I could source one. The variable buck converters (about $1/each, eBay) would solve this problem nicely and very cheaply.

The problem with the Arduino is that 16Mhz CPU operation requires 5V input for reliable operation. Lower voltages are okay but only with lower CPU clocks, and the board you are using is hardwired for 16Mhz.

Cheers
-ml