A quick summary of advantages and disadvantages of different methods ...
Console mount
Advantage is that it's located in a convenient place, along with all of your other gauges.
Disadvantage is that the length of the hose to your SPG will usually restrict the motion of the console, making use of the compass more difficult than it needs to be.
Potential disadvantage will stem from how you choose to stow your console, which will determine how readily available your compass will be when you need to use it.
Retractor mount
Advantage is the compass is easily reachable, and can easily be handled to orient in whatever manner you want it to be, often making it the easiest method for sighting the lubber line and reading the heading.
Disadvantage is that retractors tend to wear out, turning your compass into a danglie ... and they're not cheap to replace.
Potential disadvantage is that some BCDs don't give you a very good place to clip it off, and no matter where you put it, it'll be in the way of something else you'll want to reach for.
Wrist mount
Advantage is the compass is always right where you can see it, and readily deployed when you choose to use it.
Disadvantage is that learning how to sight the lubber line without some degree of "offset" due to the placement of your arm takes practice.
Slate mount
Advantage is the same as the retractor mount without the disadvantage inherent in the retractor.
Disadvantage is that you have to stow it somewhere and pull it out when you need to use it.
Potential disadvantage is that you might not be able to stow it readily, depending on how useful your BCD pockets happen to be.
As you can see, which works best for you is going to depend which configuration works best with your other equipment choices, as well as personal preference.
The reason folks are recommending the Suunto SK7 is because a compass has to be held relatively flat in order for the compass card to rotate freely. The SK7 gives you about 30 degrees of tilt before this becomes an issue. Most other compasses give you less ... some far less. The SK7 is one of the more expensive compasses on the market, and this is the reason why.
... Bob (Grateful Diver)