aerate then slit seed

 or the other way around? Does it matter. If I slit seed
do I really need to aerate also?

--
Searching is half the fun: life is much more manageable when thought
of as a scavenger hunt as opposed to a surprise party.
Jimmy Buffett

 
If you do both, definitely aerate first.  The slice seeder will place
the grass seed just below the surface, where it needs to be.   The
aerator would then just dislocate a lot of it.

How much a lawn benefits from aeration depends on the soil/turf
condition   If it's compacted soil or has excessive thatch, where
water has a hard time penetrating, aeration can be a big benefit.  In
general, if it hasn't been done in many years and you're renovating,
I'd aerate.