Well my starting comment here just has to be "You need to know the water qualities that are coming out of your tap before you even think about choosing any water filtration unit."  If your planning a Reef tank and you want it to be an enjoyable journey you Need the best water you can get!

    If you do not know the state of the water being delivered to your tap there are a number of sources that you can use to find out.  I prefer to talk directly to my local water treatment professionals over just fetching the information from the net.  There are more benefits to this than just getting the specifications of the water they deliver and leaving it at that.  It is almost amazing what some of these professionals are willing to tell you about water quality issues if only one asks them.

    One issue is that water quality from the tap varies by huge amounts over geographical areas, and often even within the same area.  Even varies over season in many locations.  The shear quantity of variables is staggering, to put it mildly.  Some areas have water that is very difficult to render pure, and I use the term "pure" loosely for "good enough" for your reef tanks use.  Other areas have water that has one or two difficult issues.  Yet others the supply out of the tap is near pristine.  For these reasons alone I cannot recommend any particular unit, nor shall I mention which I use so not to influence you.

    After you know the water quality information for your tap it generally boils down to two simple questions.  The first is Who can deliver the best water quality from your source water for the least amount of money (up front and long term), hassle, and BS?  The second is who has the better product and customer service to you?

    Due to the major differences in tap water quality there is no "one solution fits all" unit available that is also in the average reef keepers budget.  Systems than can clean just about anything from any source water exist however they are very expensive.

    At the risk of irritating someone I will say that currently there simply are not wide differences in the water treatment technologies being used in similarly priced units that an aquarist is likely to purchase.  That is not to say that there are not a handful (or two) of unscrupulous vendors of water treatment equipment that use sub par media and/or construction practices.  I simply mean that there are not huge differences between the top several vendors that those keeping reef tanks are likely to run into.

    I will say that there is a rather large difference in the methods the technologies are applied to a particular unit.  Or entire lines of units as the case may be.  The major price differences however seem to be mainly for longevity and the "bells and whistles" that some units have.  Keep in mind that most manufactures offer several different configurations to meet a wide variety of general tap water conditions.  The catch is that they are generally not configured specifically for your tap water.  The best defense is to know what your water quality is and what you need to solve any issues it has.  Then and only then do you start shopping for a unit.

    There is so much information available that anyone who actually bought a unit from one of the "bad guys", or chose the wrong unit for their individual application and can't get a refund or trade, should have little defense for their mistake.