Not the QNAP 239 vs Synology 209, but there's this thread about the QNAP 209 by comparison.
http://www.avforums.com/forums/stre...5-synology-209-faster-then-qnap-209ii-sc.html
Just an asumption there though, but makes sense. The QNAP 209 has a slower processor and less memory (only 256mb). From my experience of running my old self-built server the processor may be okay as mine's a 400Mhz CPU, but 256mb is too small if you want the stuff I want (Squeeze Centre, web server handling 4 low traffic domains, exchange-like IMAP email server, bittorrent client in the main). I've got 512mb on my self build with Fedora on it and it pushes the 512mb to the limit.
Now the QNAP 239 on the other hand is an Atom processor at 1.6Ghz and 1Gb ram. Okay the Atom is going to slow it down on some tasks, but they're very capable and extremely low powered processors. If the same CPU can run Win XP, Vista and Win7 on my Sammy NC10 netbook just fine (and it can), then it's up to the job of a simple server like this.
Problem is, being the latest technology, it's twice the price! (£400 aprox at the moment for a 2 bay model. The 439 is the 4 bay version at £600. And that's without discs!).
But for me the important thing is compact design, much lower noise than my current system, and a lot lower power consumption. Plus ease of use.
But I'm also toying with hosting my web & mail stuff externally and going for a cheap NAS just for storage and Squeeze Centre. For what I want though, the monthly hosting costs are stacking up and compared to a more expensive NAS plus leccy bill for it, over a 3 to 5 year period, it's perhaps cheaper to use the NAS (and far more flexible than a lot of cheap hosts).