Readability and cyclomatic complexity of database transaction code
It's obviously subjective, but IMHO inline conditionals make code harder to understand, indentation is very important. So I'd definitely take the second approach, but writing it differently to use only one indentation level:

existing_transaction = database.find_transaction_by_id(transaction.id)
if !existing_transaction
  database.create_transaction(transaction)
elsif !existing_transaction.processed
  database.update_transaction(transaction)
end