There are a number of fundamental differences in the technology.
First and foremost there is the density. With the standard DS14Mk2 AT, you will only be able to get a total of 56 drives (14*4) in your 12U space. With the new DS4243 you will be able to get 72 drives (24*3) into the same space. I think that this is where you are getting the significantly different number of daily deltas from. As part of density you need to remember to calculate in the cost of power and cooling. The 4x DS14Mk2 AT will produce about 1050BTU each at 3.1A each, or 4200BTU at 12.4A. The 3x DS4243 will produce about 1501BTU each at 4.4A, or 4503BTU at 13.2A. While these numbers are not stageringly different, the differnce becomes more noticable when you start getting into full cabinet solutions or multicabinet installs. Also, depending on the seismic regulations that you are subject to the DS14Mk2 solution will weight in at about 272lb (123 kg), and the DS4243 solution coming in at 330lb (149.7 kg).
Second, the SAS technology now allows for better connectivity to individual disks, and higher aggregated throughput. This is of course all negociated to the least common denominator, and that being that SATA drives are slow, and do not produce significantly high numbers of IOPS, but this is SATA for archive. So my guess is that performance is not going to be an issue.
Some of the other things that you asked about are not so much dependent on the shelf technology. While the SAS technology aloows for more drives on a "loop" you are still subject to the limits of Data OnTap. Data OnTap 8.0 is shattering a lot of hte old "rules" like aggregate size and other things like that (depending on your controller). I have seen that FAS6080 being spec'd out at handeling 100TB aggregates, and it decreasing from there.
I'd personally go for DS4243s. Key reason: they are newer technology as you pointed out.
The beauty of NetApp modular system is that you can move building blocks in & out as & when you need. So I bet at some point in a future you may be looking at buying new heads for shelves purchased today. And for obvious reasons whatever the spec of that box will be, the likelihood it will fit nicer into a combo with newer shelves is higher.
Re capacity & density - I believe 2TB SATA disks have been just released. Did you take them into account in your calculations?
So is there any official speeds and feeds from Netapp on this ? I agree with Radek and have started configuring SAS drives/DS4243 instead of FC/DS14 in alot of cases. But there is a perception from the customer that SAS is slower, and it would be nice to have some factual stats to discuss with them, not just looking at the SAS disks but the interfaces, loops, etc. - al the clever bits that Netapp have come up with to make it perform so well.
The only real advantages out of performance are that with the SAS loops there is a much lower likelyhood of hammering the loop speeds, but to be honest, you aren't going to be doing that anyway!
The DS4243 is new tech, but it is pretty tried-and-tested tech in regards to the whole industry. Second to this is that the DS14's will go EoL at some point, so as far as future expandability goes, you'll get more life using DS4243, and with future disk drive technology, you'll again get better support for these shelves.
I would go with the DS4243's, I have 196 in prod now...I like them though I have had one major issue worth mentioning.
There are limitations you may encounter depending how far you cabinets are spaced about in your DC... There is a max of 5m (16') SAS cables. a solid wiring diagram will get you past this and I understand NetApp is working on a 10m cable.
The issue I encountered:
A new issue as of 7.3.2P3 (burt 401176)... By default the ACP communicates via SSL, if you have secureadmin (SSL) configured and or OpsMgr is set to communicate over SSL there could be a race condition between ACP and the HTTPS which will cause a panic (what happened in my case)
There are a couple workarounds to this burt, disable https or disable ACP or both. Given that DS4243 engineering recommends the use of ACP, they suggest disabling https (options httpd.admin.ssl.enable off) first to prevent the panic. In the event of a recurrence, they are okay with disabling ACP (options acp.enabled off) too.
Anyways, thought the community might like to see this.