Ok, well lets start with windows 7 expiration
windows 7, being a beta, has a build in "time bomb"
this means that, come august 1st, you will not be able to use windows 7 anymore, because you cannot upgrade it past the point it is now, i dont know if it will even boot.
as for 64 bit vs 32 bit
The fact is, windows 7 is set to be the LAST 32 bit operating system, almost every new computer on the market is 64 bit, notice i said ALMOST every one. Compatibility issues with 7 are present, but it is still beta, most vista drivers will work, and companies like nvidia have drivers for 7 already.
the big jump from vista to 7 is in the middle and outer workings of the operating system so far.
inner workings are the kernel
middle workings are the services that run the computer and help to control it
outer workings are the user interface
microsoft wanted to get the most talked about issues first, going for the user interface and speed issues and fixing UAC (nice job on that BTW)
The next big problem windows faces is the kernel size, they want to decrease the size of the windows kernel, making overall speed increase at amazing rates and startup / shutdown times enhanced.)
they are also working out some stability issues that originated in windows vista due to ram errors as some of you may noticed, upon the installation of vista x64 with a certain amount of ram you would get a blue screen during boot, fixed by removing one stick of ram until installation was finished.
as for my preferance, i like windows 7, BUT me and 5 of my friends, having all downloaded it at the same time, adding it to our vista systems as a dual boot and running it, awesomely for a while, all went back to vista, i cannot say why, it may be that all of our programs are on vista, or that some wireless cards tend to stop working after an hour or so and the machine needs restarted, im not sure, but...
VISTA X64 ALL THE WAY!