Do you have any other cordless tools? If yes get the one that shares the same battery.
Almost all my cordless stuff is 18v Dewalt. Yes I have the dewalt impact shown by another poster. I like it but it doesn't have the punch of a air impact.
If you don't have any other cordless stuff but think you may in the future pick a line that has a fair bit if variety of tools for that voltage and brand. As another said if it will only see infrequent use and will be your only cordless tool just get a air or corded electric.