Battery Power for Arduino Pro Mini - 3v3 or 5v?

I'm going to start a simple blinky LED project that will be embedded in a hat. All my current Arduino stuff is the larger 5v system. However I can't seem to find any common small batteries that give 6-7v, and a 9v is going to be way to big. All the small coin batteries are ~3v.

So I think my decision is, stack 2 coin batteries, or get the 3v3 version of the Arduino Pro Mini. All I'm going to be doing is fading 9 or 10 on and off LEDs, so I'm guessing the 3v3 version should do just fine. I'm assuming the PWM pins and everything works similar on the 3v3 even at 8MHz.

Any thoughts on which direction to go?

Giving 6VDC to the VIN on a 3.3V Pro Mini sounds like the way to go. The voltage regulator needs some room. You'd need to give about 7.5VDC to the VIN of a 5V Pro Mini, or the regulator would be spotty. Remember that batteries drop voltage as they are used, so design up from the "dead" voltage, not down from the "full" ideal voltage of the battery.

For this sort of application, you might consider bypassing the regulator and running your 3.3V arduino directly off of (for example) 3 AA batteries (providing 2.7 to ~5V, depending on battery type and freshness.) There probably isn't anything in your circuit that NEEDS a regulated voltage...

After some more digging, I also found this great thread:

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1223295094/all

I've gone an ordered the Arduino Pro mini 3v3 version. So when it gets in I'll post with powering results.