Honestly, I probably wouldn't use either as a first name. Neither sounds like a name that is likely to do very well professionally. Although, for a first name, Cash is probably the better option. While not particularly common, Cash ranked 263 in 2011 and has been listed on the Social Security Administration top 1000 name list since 2003. So it's not likely that others will find the name quite so "out there." One woman I know chose to give her son a common first name (William) but used Cash as a middle name and called him that. She adored the name cash, but wanted to give him a more professional sounding first name to make things easier on him as an adult. Ashby on the other hand has never been in the top 1000 baby names according to the SSA (not that there hasn't ever been a child named Ashby in the US, it's just an extremely uncommon name). I like the name, I think I actually like it better than Cash, but it is only a one letter difference between Ashby and Ashly which is commonly used as a girl's name...which brings to mind the Johnny Cash song "A Boy Named Sue." It's likely to be an easy target for teasing. I would definitely stick with Ashby as purely a middle name mostly for that reason.