I really like this question. I can tell you what I know, but unfortunately it is far from a conclusive answer.I have not seen any solid performance numbers comparing these filesystems, only conflicting claims. I can tell you that the performance of ext3 does degrade when directories contain extremely high numbers of files, whereas UFS does not have that problem.
On Linux, for this purpose, I would definitely consider reiserfs rather than ext3. It does journalling, and is noticeably faster for lots of smaller files that change frequently, and also does not take a performance hit on larger directories.
More significant than performance, though, are the features. One feature of FreeBSD that is very nice for a NAS is its ability to take a ghost snapshot of the filesystem. This can serve various purposes, one of which is to do a filesystem check on the snapshot, without taking the server down. If the snapshot is clean, then the filesystem was clean at the time it was taken. Saves downtime, while giving you that confidence of a clean filesystem.
On the other hand, to my knowledge journalling is not yet stable on UFS, and if the filesystem is dirty or damaged and needs repair, the system must come down, and that recovery will be much quicker and more reliable on a journalling filesystem like JFS, ext3, or reiserfs. It can make the difference between a few minutes or an hour or more depending on the size of the volume, and the extent of the damage.
As I understand, the journaling feature of ext3 was based on the JFS code. Reiserfs's journaling is quite different, though it serves the same purpose.
FreeBSD/UFS has a number of other interesting features that are not found elsewhere. One example is the ability to set a file to be only appendable, so that you can add to it, but cannot change data already in it.
The other consideration is the OS itself. While eCS can serve this purpose, I would not choose it. Firstly, it takes up an expensive license that could be used for a nice desktop, and only partially utilizes it. Secondly, unless you expend considerable work trimming, it uses more disk space than a minimalist Linux or FreeBSD installation, wasting space on a system whose sole purpose is to be disk space. Third, it is not as easy to set up backups and recovery procedures.
All of that can be dealt with, and there may be reasons to choose OS/2, e.g. it may be easier for you given your familiarity with OS/2. It might be worthwhile, though, in this case, to look at some backup/recovery programs and scripts used in FreeBSD or Linux and apply them, create yourself a custom recovery CD, and most importantly of all, post pictures, screenshots, descriptions, etc. on os2world.com.