ZFS boot with two disks

[Joshua Isom]

I'm wanting to set up a simple home file server and basic dev box with 
FreeBSD.  I have two 500GB disks that I want to put in it, one of which 
is empty after cleaning up.  I hope to use gpt and zfs for the disks, 
but I'm unsure about some parts.  A lot of the stuff referring to 
booting doesn't mention concating disks(I'll take my risks) for root.  
I do want the second disk bootable as a fixit disk.

For a fixit disk, is it better to use UFS or use ZFS for consistency?  
If I use two pools, one for booting and the other for data, how much of 
the system needs to be on the booting pool?  I'm hoping to set up the 
disks before putting into the new computer, so will adding the second 
disk's space to the pool cause any problems?  Everything I've looked at 
talks about single disk booting or using a raid just for data storage, 
so this seems to be a lesser used approach.

And finally, would it just be better to stick with tried and true UFS 
with the MBR or are the benefits good enough?