The problem with linking their compensation to your success is how do you measure success and when does the meter shut off. 

In my opinion the only way an investor can really control their fees is by using the fixed rate per transaction.  You know what the total fee is upfront and you have full discretion over how much or little you incur in a year.  Any investor who is put in a position of feeling like they are being pushed to make trades should find another adviser (or gain the knowledge to manage their own affairs).

Percentage based models usually mean the meter is continually running - win or lose and regardless of how much or little work the adviser is actually doing.