A little leg work on your part would have revealed a fair few posts that would help make up your mind:
http://www.linuxquestions.org/questi...d.php?t=505337
http://www.linuxquestions.org/questi...d.php?t=416849
http://www.linuxquestions.org/questi...d.php?t=450532
And those are only the 3 most pertinant that *I've* posted in.
However, to answer your questions:
Q: Is 64 bit linux noticably different?
A: Depends. For Firefox? You loose out on flash and java. For ModelSIM and Matlab? You gain a lot of speed. As far as "user experience" goes, slamd64 is Slackware but recompiled.
Q: If you decided to change to Slamd64 from Slackware what motivated you
A: I felt I wanted to use my 64-bit hardware and i had some free time. If i was making the choice now without knowing what i do now, i'd hesitate; but if you're prepared to learn some extra tricks (see above posts) it will reward you. I use it at work (11) and at home (10.0-custom fixed thing).
Q: and have you been happy with it?
A: Most of the time. In the past there have been some annoyances that have needed some pretty dodgy fixes, but that's more to do with some old binary packages I run than Slamd64 itself. On occasion i do get narked at having to install from source, but that's only due to time constraints, once set up, it's set up. And fwiw to date I cannot think of anything, off the top of my head, that I haven't been able to get working, 32 or 64 bit.
qparted your drive or use qemu and test it on that because in the end all you'll get is opinions from users who likely won't use it for the same things / in the same way as you do =)
The real big question is going to be "What do you want to use it for?", so having a little checklist of "things that must work" is probably not a bad idea when testing.
Hope that helps, visit the slamd64 forum if you want to see what sort of issues they have there (mostly the same as they do here).
- Piete.
[Edit]
The first line sounds a bit confrontational, but i dislike depublishing so i'll add an apology here if you did look but failed to find anything of value. Much of the internet is drowning in data but starving for information, and it's easy when you've been there before (ie: posted in the thread) to remember what to look for.