Not all systems support both APM and ACPI. I switched because my system only supported ACPI. Pretty easy decision really. If you're switching to get S3 (suspend to RAM) support and you're using a 2.4.x kernel, don't bother. It is not supported. Period.
Not sure if your system is supported? ACPI4Linux has a list of supported machines/BIOSes started on their Wiki. Please contribute to the list if you've installed ACPI! They also have a list of machines that are not supported.
For more information about the power management in laptops you may (also) find the Battery Powered Linux Mini-HOWTO useful.