Jisho shows 花金 for the main dictionary entry and 華金 as an alternative, so I believe 花金 is more common, but as the other comment says both kanjis have very similar meaning so they both should be ok.
A bit related to that, I heard that you are restricted on what kanjis to use for names but not on how they would be read. So a person could be named 空 and have the reading be スカイ(sky)