We're due to replace our 2 x 3140 clusters in the next few months. My US collegues have suggested a single 3270 cluster, my thoughts were for 2 x 3240 clusters instead. The cost is pretty much the same all in.
Based on the published SPECsfs one 3270 is barely more than two 3140, and we're likely to be keeping this config for 4-5 years.
These filers will be serving out about 30TB of SAS/FC to VMWare and Exchange and about 70TB of CIFS for 700 users.
Check licensing very carefully...last I remember, licensing was different between tiers. Though a single high tier against two lower may equal out.
Most importantly as Radek stated, a single head has no fail-over, can you be without a system for the down time required to get parts/service? Whose job is on the line if that downtime becomes "problematic"?
Having less boxes to manage makes life a bit easier.
However, there are still certain operations in ONTAP which are bound to the so called Kahuna domain & can't be properly multithreaded - so two boxes (four controllers) can deliver more oomph in certain circumstances.
Also, if for whatever reason a filer goes pop, two are better than one, as each runs only half of the services.