The 32bit and 64bit builds use different versions of libtorrent.
The 64bit build is a community build, the 32bit build you get from the http://www.qbittorrent.org/download.php
downloads page is the "official" one
I do however agree with you, an "official" 64bit build would be nice.
I would go even further - The x86 version should not even be recommended, because that version of qBitorrent has some huge issues for anyone with a remotely fast connection.
Even before the whole I/O errors and subsequent crashes, I experienced the freezes and crashes like many users (such as the OP) were reporting. When I loaded 600+ torrents and speeds reached 120Mbps+, it started crashing like crazy. It was driving me nuts given the fact I was very limited on my choice of clients (because of some private tracker rules).
I even though I was doing something wrong, but when I saw dozens of users with the same problem as me...
The x64 version is infinitely better. Right now, in my opinion, it's the best torrent client there is, where the x86 was by far the worst client there is.
If the x64 version works with every modern OS and CPU, why shouldn't it be, at the very least, the recommended version of qBitorrent? Right now I'm seeding at 200Mbps with 615 torrents with no problems (and doing that 24/7 for 4 days). That would be impossible with the 32 bit version of qBittorrent.
Is there a reason for it not to be recommended?