I don’t think performance is a criteria, unless you can point to one drive (say hda) and say that drive is 10x faster than the other drive (say hdb).
If performance is very important, then < me ducks the flames > you should be looking at a very light weight distribution such as elive or someother light weight distribution, and not openSUSE, nor Ubuntu, nor Fedora, nor Mandriva, nor most other distributions. For sheer performance in software, then IMHO light weight is what is important.
Rather when choosing what goes on hda and what goes on hdb, I look at risk of installation, and ease of maintenance (keeping future updates in mind).
If one has both OS on hda, where hdb is only /home/username/data, and if one wishes to replace hdb with a faster/newer drive, then it is relatively easy. One edits the config file (/etc/fstab) to comment out the old specific hdb entry, one inserts the new drive, one boots openSUSE and using the appropriate tool format it to ext3 (soon to be ext4 in openSUSE-11.2) and then one edits the /etc/fstab file (this can be done via a tool called YaST) so the new drive is identified.
If one has openSUSE on hdb, this is more difficult, as when hdb is removed, one loses openSUSE.
If one has grub boot manager problems, I find having both MS-Windows and openSUSE on hda easier to manage with workarounds. But thats just me and my limited grub knowledge.
Some users like the idea of separating openSUSE and Windows on separate drives, as they believe it reduces the risk of their losing MS-Windows during the openSUSE install. I don’t share that view, although I understand their sentiment. One does have to be careful during the openSUSE install to ensure it does NOT over-write one’s MS-Windows, or that it does not take too much space away from one’s MS-Windows.
But again, these are not performance issues, but are rather installation and maintenance issues.