It really depends on your business needs. What you seem to be leaning to is leaving one controller pretty much as a backup, which is fine if that's what you want to do. My only comment is that on controller 2, you can't not have a spare; Ontap won't like it, so go with the 2-disk raid 4 aggregate and one spare.
TR4070, Flash Pool Design and Implementation Guide, is interesting reading and you can get it from the support site.
