In my opinion there are a few major philosophical differences that may make you lean more to one or the other. I would not say that either is better but rather they have differences that change your preference.
For a managed account what you really are investing in is the manager and his or her portfolio methods. The norm is dollar cost averaging and a long/short weighted periodic re-balancing of the portfolio. However, there are various risks that come along with investments in say mutual funds. They are much more rigid than ETF's and may be more difficult to move in and out of among many other things. That is probably the biggest kicker in my mind.
However, ETF's work like stocks and can be shorted where mutual funds cannot. They trade just like stocks to put it in simplest terms, and are specifically designed for very defined goals. Often these are achieving some mathematical benchmark of say 1,2 or 3 times a given index performance in either a positive or inverse correlation. The big kicker on these is that often they are 'reset' so to speak daily, weekly or on some other time table. So if it is reset, this will greatly affect the volatility and can create windfall gains or losses if there is a large movement. They do not fit a typical buy hold but can be blended with mutual funds and held in a managed account to add diversity and so higher yield. So a word of caution, you need to research them properly. A great source I have found is sites like seeking alpha and similar. They do a good job of providing detailed info on each. Best of luck. If you got other questions let me know.