If by RAID10 you mean Linux's "fancy" direct RAID10 driver (with its numerous potentially handy tweaks) then one factor that may affect your decision is that most installers for stable distor releases at the moment do not support creating such arrays at install time (and may not have the driver present to install onto a pre-created array if that type). Most can create and install onto a traditional RAID1+0 nested arrangement though.
I would suggest a separate small array for the base OS if the budget for this is not an issue, especially if you expect the machine to need to swap (though I'm guessing you have specified that much RAM in order to reduce, if not remove, the need for paging to disk) as the separate drives will mean application/data IO activity and the kernels swapping activity will not be competing with each other for time in control of the same sets of drive heads. Also if something drastic happens and your large array goes properly offline, having the OS separate may make recovery attempts a little easier.
Performance wise I would guess that SSDs would be overkill. The base OS portions are not going to be read from disk very often as they will spend most of their life in cache (unless I am wrong in assuming you do not anticipate much, if any, paging RAM to/from disk type activity). You might be better of moving the extra cost into other parts of your budget. Having said that, the SSDs certainly won't do any harm so if the question is cost neutral or there abouts (i.e. the SSDs would cost little/no) and you don't need the extra space offered by the equivalently priced spinning disks you might notice a little performance boost (though really, your other arrays are were the main I/O action is going to be). And if do you anticipate an appreciable amount of swapping activity then small SSDs would definitely be better than larger spinning disks (though make sure you get models with good random write performance - some SSDs are not as good in this respect as they are generally expected to be).