Level 6 follows the optional rules for deductions (though I think there are some minor adjustments), so they are expected to hit handstands on their bar casts, even though their SV requirement is only past horizontal. They are also expected to hit all their splits to 180* on beam and floor. I believe L5 is 150*. Doesn't sound like much of a difference but there are a lot of girls in optionals that get deducted for not hitting their angles. These deductions have a range, so if the gymnast is close, she will be deducted less than one who is way off (theoretically, of course).
As Gracy said, L6 gives more options for making the routines easier/harder for particular gymnasts, though I wouldn't say that L5 REQUIRES harder skills. Many gymnasts would agree that a cartwheel or roundoff on beam is *much* harder than a BWO or BHS, my dd included. She learned a BWO just to avoid having to do a cartwheel on beam in her silver year (xcel).