In general discussion, higher battery voltages often have more working power and can last longer.
But the real question is how often will you really use this, and for how long?
If you are a weekend warrior, with occasional prjects. Go with a Craftsman or Skil.
If you are going to be giving these a workout, DeWalt and similarly priced models are the way to go.
Most of these have adjustable RPM. Low for metal, high for wood.
Weekend Warrior mayself. I buy a lot of my "occasional" tools from www.harborfreight.com , serious stuff I go for DeWalt. Am happy with both choices.