Price difference is negligible for win7 home or pro if you get the OEM license. We're talking about maybe $30 difference when they're on sale, less than a tank of gas or case of good beer.
Pro has features like backup to network storage instead of just locally attached drives, remote login, drive/folder encryption, etc. And *eventually* you'll probably end up with win10 anyhow, so you may as well get the win7 pro to make sure your upgrade translates to win10 pro.
There aren't any downsides to pro vs. home, except the marginal cost difference. As for 64 bit vs. 32 bit, there's no reason to not get 64 bit and if you go 32 bit you're stuck with no more than 4GB ram.
Every one of my computers that haven't gone to win10 already has win7 pro. I had one computer that I was building "cheap" and I got a steal on win7 home 32 bit, but the little features that weren't available bugged me and then I had one session where I exceeded 4GB memory use and it started swapping to the hard drive, so that computer got a fresh install of win7 pro the next day.
As for winXP mode... Its easy to automatically set it up with win7 pro but in my experience it totally sucks compared to the latest free versions of VMware. I tried winXP mode for a handful of programs and old games that won't run on win7, and almost none of them worked with winXP mode. Most DID work with VMware using images other people have built and put out there for download, and the rest ran on dosbox. So you don't really *need* win7 pro for virtual machines unless you just want to play with winXP mode.
I finally only ended up using winXP mode for TOR, but even on a fast core2 quad with 8GB ram, the winXP mode was slow as heck. Horribly slow, and I couldn't find out why. It won't use even 25% of a single cpu core, and I spent a week or two trying to chase that down and never could find out how to give more cpu cycles to the winXP mode VM. So that made using TOR in the winXP VM painfully slow. But, I wanted to try out a few things without risking my main machine's drives and installation, so using a VM was the obvious choice.
