the vanilla 409
Can take either 3.5 or 2.5 disks and has very similar noise levels, but a bit lower - probably because they can fit larger, slower fans into the case.
The power consumption on the 409 is similar at idle (16w for the 409 versus 12w for the slim)
If only the Reg editors would ask their hardware reviewers to study up on benchmarking methodoloy or else focus on the usability and cite more thorough benchmarks than they can provide. benchmarking a nas properly ... it's a complicated process and the benches here provide precious little info, as is invariably the case in hardware reviews.
My tests of the 409 gui leave me very disappointed. It is much prettier than the Promise GUI, yes.
However, all that enabling NFS support does is start the NFS daemon - there is no option to work with /etc/exports given (that I could see) in the GUI in either simple or full mode. The only editor available at the commandline by default is vi. Asking a newbie try to work in vi and then try to get a working exports file going is just atrocious. At least the Promise NFS gui actually did ask "oh, um, who should have access to the NFS mounts?" and then set up an export, even.
Raid level migration is similarly murky. Expanding a raid from 1 disk to three in raid 5, you are not shown which disk will be used as the data source, you have to trust the device that it is in fact going to erase only the newly added disks. Very pretty but very uninformative as to which disk is where. Or how many total you will be left with.
If only the original promise 4300 weren't such a noisy beast! The 4600 is somewhat tempting, but does not give you what Synology does, real root access to the box and a package manager.
If I were recommending one of these to someone who wasn't a pro, though, I might recommend the gen 2 2disk Promise system in preference over a Syn 209, because while ugly the UI is actually more functional. I will wait and see to find out how loud the next generation of Promise systems are. The claim is that the 4600 is fairly quiet and fairly fast.