Should I buy the 104?

Synology's interface is just wow, that's a fact.
OS6 had some problems with stability but this is actually being fixed by firmware updates. You would have asked the same question 1 month ago I would have said avoid ReadyNAS, now they are perfectly usable. The interface is classical but pretty simple to use. I still miss some things Synology has like a firewall or a mail server but the price is not the same, got 239€ for the RN104 (which I've often seen discounted) and 299€ for the DS413J. The DS413 comes with 1.6Ghz though meaning it should be faster but only has 1Ethernet port. That's up to you to decide.
RN104 gives 90-95Mbyte/s read and 40-50Mbytes/s write with 4*4Tb seagate NAS HDD raid 5.
The torrent app is there but I believe Crashplan will require more memory than what either of those models can provide. The RN104 memory is NON-upgradable. I don't know for DS413J but I believe it's the same.

you can test the interfaces of the Netgear : http://apps.readynas.com/pages/?page_id=143 and the sinology http://www.synology.com/fr-fr/products/dsm_livedemo

As a CrashPlan user, I strongly suggest an x86 NAS with a minimum of 1 GB ram (preferably 2 GB or more) - no matter who makes it. http://support.code42.com/CrashPlan/Lat ... quirements

There is no CrashPlan app, but it is not difficult to install with ssh.

You are likely to hit performance issues with the 104 if you are doing a number of things against it at once. With 9TB of storage (I'm guessing RAID5) the device will be tracking a large amount of general disk meta data - the directory and block structure that defines the file system. If the device is support single user tasks the memory it has available for data is OK, but the cache becomes overloaded if there are a number of tasks working in different areas of the file system at once. The same is also true for the CPU, as any operation that does not get blocked by the disks will fully load the CPU.

As for BTRFS, Netgear are currently cheating as they use BTRFS on top of standard RAIDed disks, rather than leave BTRFS to do the raiding work. The reduces the number of 'new' features that have to be perfect, while leaving the same old problems of RAID setup/rebuild times.

The 104 makes a nice little single use home device (which is what I have), but if you plan to do a lot then the 413 would be a better option because of its memory and CPU.