There is no difference between tower and rackmount cases, except price.
Both have adequete cooling, both can have redundant power supplies or not, etc...We know what we are talking about, we run a facility with nearly 2000 servers, a good mix of both tower cases and rackmount cases, and we see performance of both kind. Each kind has junk cases and good cases, and they are not good cases just because it is "rackmount", and not junk cases just because they are "towers".
The only consideration is the footprint (space each takes up) because some providers will only host rackmount servers. This is prevalent in carrier hotels, where space is a a premium. Otherwise, most providers these days do take towers as well as rackmounts, though some do charge more for towers (because they can).
The only important things are adequete cooling and good power supply, which can come in tower or rackmount case formats...
We happily accept towers for co-location:
http://www.unrestricted.net/colocate.shtml
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