advantages to an upgrade: everything moves along with it (accounts, apps, data, domain info, settings, etc.), very little interaction or questions to answer (other than “do you want to upgrade?”)
disadvantages to an upgrade: slower than a fresh installation (although everything is preserved after the fact and it’s typically automated), any problems are possibly still problems after an upgrade and it’s possible problems are introduced this way (unsupported software, hardware drivers not supported by upgraded version, etc.), requires same media for the upgrade in most situations (if you are running windows 7 beta ultimate 64 bit beta you have to upgrade to windows 7 release candidate ultimate 64 bit)
advantages to fresh installation: real world scenario, nothing is upgraded (for better or worse), start fresh (if i no longer use a particular application or utility, it’s not running on my system taking up space at a minimum)
disadvantages to fresh installations: have some work after the installation (join the domain, installation applications, data transfer, customization, identifying networks, etc.) although initial installation is quick, in the end it is typically the same or more time than an upgrade.
what about boot to vhd?
boot to vhd has so many advantages and has truly changed the way i have been working with windows 7 and the interim releases. this is not making a commitment to running windows 7 permanently on your system – you can still boot into your current operating system. this also means that it is most like a fresh install – you have to install your applications, join the domain, etc. if you want to use it for work. you can access the “real” hard drive from within the boot to vhd, so you don’t have to make a copy or move all of your data. not having to constantly rebuild the real system has been a great productivity improvement and being able to quickly confirm things are working before committing to an upgrade or fresh installation has been great. i used this with my parents computer to allow them to test windows 7 and get a feel for whether they were interested in upgrading to it from windows vista immediately at the release candidate or waiting for the final version. my dad is sold (my mom doesn’t like change... period) – if they ask my opinion will upgrade their “real” computer to windows 7 when the full version is available migrating everything along with it. if they have issues, i'll do a fresh installation.
the real benefit of boot to vhd is that you are able to experience how windows 7 will work on your real hardware before committing to a rebuild (fresh installation) or an upgrade. the focus of the release candidate is to provide as much “real world” input as possible and upgrading from Windows Vista to Windows 7 is real world. if you installed a boot to vhd of the beta build and are still running windows vista, upgrading windows vista to either the release candidate or waiting and upgrading to the final version is still an option.
the choice is yours...