This one isn't a myth: it is quite possibly true if your firm is having real issues with missed dates, a poor follow up system or terrible document saving procedures which means no one can find anything.

Case management will yield benefits for a firm that has issues regarding, well...the management of their matters - tracking, critical dates, follow ups and so forth. Document management will yield benefits for firms that have issues with saving and finding documents related to a specific matter, or finding a precedent that meets their current needs.  Document assembly will yield benefits to any firm that creates many documents from day to day. Each of these three investment areas provide benefit in varying degrees, depending on the firm's current issues and requirements. Whilst they are not what you would call "totally independent" of each other, each of these areas is a distinctly separate approach to profit and efficiency within your firm.

Short answer: there is no single avenue that is "more profitable" - its horses for courses. Where do you feel that YOUR FIRM is losing money, or failing to make more of it?  My question is always: Where do you spend most of your time? Is it:

1. Tracking critical dates and bringups?
2. Looking for old documents?
3. Drafting, proofing, finalising and producing documents?