Paul Thurrott's SuperSite for Windows has a look at Windows Vista SP1 RC1 and Windows XP SP3 RC1 overview and screenshots:

Vista SP1 RC1: I haven't been using Windows Vista Service Pack 1 Release Candidate 1 long enough to be able to determine its fitness, but previous pre-release builds have been notably stable, and I don't expect to see any major problems this time around. What I'm more concerned with are the perceptions around this update. Folks, Windows Vista is already a stellar operating system, and whether you believe that or not, please don't be misled into believing that SP1 is going to change the overall experience very much. Even in pre-release form, SP1 is solid and stable, but it's not a big change for anyone who's been using Vista all along. That said, this is the latest version of Vista. If you're running this OS, you're going to want to check this one out.

XP SP3 RC1: There have been reports online that SP3 improves the speed of XP by about 10 percent over SP2. I haven't seen any performance improvements personally, but then I have only upgraded a single XP SP2 system so far and have no plans to run this configuration in any day-to-day use. Certainly, XP is faster than Vista on the same hardware, regardless of which service packs are installed. But that's the nature of upgrades, and Vista more than makes up for any benchmark deficiencies by providing far more important functional and security improvements. XP SP3 supports a Product Key-less install option, as does Vista (and Windows Server 2008). I have yet to slipstream SP3 into XP, but will do so to both test this option and to document the XP slipstreaming process for a final time. However, I know that XP SP3 does not install newer versions of Internet Explorer or Windows Media Player. So if you were to slipstream XP and SP3 together, you'd still have to upgrade from IE 6 to IE 7 later. For the few people running x64 versions of XP, XP SP3 does not apply to you: Your OS is updated via Window Server 2003 service packs. The latest Windows 2003 service pack is SP2. In actual use, XP SP3 doesn't really change the XP experience, but then that was the plan all along: As with Vista SP1, XP SP3 is envisioned as a "true" service pack that rolls up previous fixes and adds just a few minor new features.