Stevy,

In formulating my answer I presume you are considering colocation or some kind of in-house DC. What you decide to do with your cases depends almost exclusively on the cost of real estate.

Tower cases are expensive with respect to real estate cost. They are big and you can only get three across into a standard rack and a maximum of maybe 15 into a full cabinet. Contrast that with getting, in practical terms, 35 1U rackmount cases into the same colo cabinet and you see the difference.

The downside to rackmount cases is that the cases themselves are insanely expensive (relative to the cost of a tower) and can be a pain to mount if you have to do that yourself. The second problem with rackmount cases is that you have to be very aware of airflow in the case itself. If you don't take great care to make sure you get enough air volume through the case (and the cabinet if you have a closed cabinet) you will start to burn out components.

So, if real estate costs (colocation or DC footprint costs) aren't too expensive or if you aren't locating a lot of equipment then a tower case is probably your best bet. If you're dropping equipment into a very expensive DC of you want to get 30 into a cabinet then a 1U case is the way to go.