There are a lot to be considered.
1. You need to first decide the vendor from which you are buying hardware.
2. You need to judge them on the following parameters.
a) Do they have propietory OS?
b) Do the native OS has the native high availability functions – Whether that comes bundled or additionally charged.
3. How strng is their storage technology is?
4. Are you looking for something open system like Linux?
5. You choice then limts only in three (Redhat, Suse, Oracle unbreakable).
6. I do always prefer Sun with Solaris and HP. It is better to assemble some roadside hardwares than to deal with IBM. Seriously and ferociously IBM hardware shouls be the last choice. None of their support and product is strong, especially if storage (lifeline for any SAP landscape) is concern. Except AIX nothing is good as per as IBM is concerned…It includes even tivoli, if used for monitoring solutions.
7. This is the last and lucky question…You should also think about the availability / portablilty of virtualisation…