Classic works fine with X on a UFS volume. You just can't have both on same and you really don't want that setup anyway.
Safety is that Norton etc won't trash the volume directory and drive. Unix is well-known and HFS+ is a newcomer on the block.
Just be sure to initialize with latest Drive Setup and if you have Classic, X, and Main volumes and store all your data on Main volume, you don't have to worry about access to data and files, and you can copy files.
Also makes it easier to wipe volume to install 9.1 later or an updated X on their own volumes.
Apple doesn't want to push users away or scare them. OS X feels snappy enough, but that could be after I disabled most of the extensions in Classic, 3rd party and Apple, that don't do anything, aren't necessary, etc.
If you want to use Mac OS X's BSD underbelly then you will be interested in having a partition that's UFS based. UFS is case sensitive (not merely case preserving) so you can have "foo.bar" and "FOO.BAR" in the same directory, whereas HFS+ can not.
HFS+ is really the way to go if you are coming from a Mac heritage and you want the best "user experience". It preserves meta information that UFS doesn't. It's case insensitivity will alphabetize a list of files the way a user expects:
And as others have pointed out it is the best format for compatability with Classic and Mac OS 9.
Unix geeks will like UFS
Mac novices will like HFS+
If you're not certain which you are, I'd make an HFS+ partition for your boot disk and an auxiliary UFS partition.