First of all...wait! Apple itself is
rumored to be ready to introduce a home server of their own at MWSF. While I have strong doubts that this will really happen, I think it's worth waiting a week to know for sure.
Secondly, HP have just
announced an update to their MediaSmart servers, which should be available in early January. This is the reason that the current models are on sale. You can read a fairly comprehensive overview of the new models
here. Of special note is the enhanced support for Macs (Time Machine backups). HP have reportedly confirmed that the Mac backup functionality will be made available to existing MediaSmart owners via a software update, so you might want to look into this if you're considering picking up one of the older models on sale.
Regarding your server-vs-NAS question: I run both--a Dell PowerEdge/ Win2000 server and a ReadyNAS NV+ NAS--but obviously not the particular systems you're looking at. Based on this background, however, I can offer a few perspectives. The main thing to consider when evaluating this choice is how much flexibility you require. NAS boxes such as the QNAP or ReadyNAS are, in fact, specialized servers, built on a proprietary embedded OS. Bug fixes and increased functionality are provided, occasionally, via firmware updates; additionally a limited number of "add-ons" (essentially, applications written for the particular box) are available to provide other capabilities.
By contrast, an open server platform such as WHS should be much more extensible over time. WHS is based on Microsoft's enterprise-class server products (presently Server 2003, but future versions are rumored to be based on Server 2008), so it in fact is built on an extremely reliable and stable core. Yes, there was a bug in the initial release that caused data loss in certain circumstances; this marred an otherwise solid debut for the product, and has since been fixed. Don't think for a minute that
any platform is immune from this sort of thing--ReadyNAS owners have suffered severe data loss problems as well due to bugs. Your "leeriness" of Windows is perhaps understandable, but unnecessary here--MS's enterprise products are far more impressive and stable than their desktop OS's.
What I think you will see moving forward is that many more applications and extensions will be written for the WHS platform, so my guess would be that you'd get a better return on your investment over time with WHS than with a proprietary NAS box. If the choice was, say, between a NAS and a box running Windows 2003 or Small Business Server, I'd say that going with the NAS for simplicity and ease of use would be a no-brainer. But WHS has changed the game considerably, by combining the power of a full-blown server OS with the ease of setup and use that one finds in an "appliance" such as the ReadyNAS or QNAP. I think, particularly if more hardware vendors jump on board with the "home server" concept (e.g., Dell, or even Apple with an OS X-based offering), that these home servers will drive proprietary NAS boxes out of the home segment. Certainly Microsoft and its partners, as well as Apple, have the marketing power to bury the smaller hardware outfits producing NAS boxes. Time will tell.