SheepShaver emulates a PPC Mac, BasiliskII emulates a 68k Mac. Both can run 7.5.3, but if the application you need is 68k only it will probably run better in BasiliskII.
Regrettably, the site that contained the only available setup instructions for BasiliskII MacOSX was unexpectedly removed and no one appears to have saved a copy. To get an idea about how to proceed, you can study the setup guide for BasiliskII Windows and the setup guide for SheepShaver MacOSX. Though not identical, the basics are similar.
The ROM file you need for BasiliskII is a different one than the ROM files used for SheepShaver. Look for quadra650.rom or performa.rom. Configuration for BasiliskII is done with the BasiliskIIGUI application that is provided with the BasiliskII application. Terrible interface, but it works. Settings are saved when BasiliskII is started with the Start button in BasiliskIIGUI.
An alternative is the BasiliskII port by Nigel Pearson. The emulator itself has several problems, but it has a prettier and more intuitive interface for configuring the various settings. After you have finished setup, you can then use the more stable BasiliskII version you downloaded here. (Both ports of BasiliskII use the same preference file, the file ~/.basilisk_ii_prefs which is invisible in the Finder.)
BasiliskII port by Nigel Pearson: http://web.netcall.com.au/~pear_compute ... iskII.html
I wrote that the invisible preference file is ~/.basiliskii_prefs, but it is ~/.basilisk_ii_prefs (corrected now in the above text).