So far I've owned tarantulas and a scorpion. To me, the care for both different inverts is equally simple. That might depend on how complex you make the enclosure, the personality of the individual species, and where it's native to.
If you make a beautiful display enclosure, it may take a little more work to keep it maintained, although inverts don't really seem to be that messy. As far as tubs/kritter keepers go with only a couple cage items, the maintenence is pretty simple.
Many tarantula species can be pretty flighty and quick which may be a bit trickier when it comes to feeding, watering, misting, spot cleaning, etc. The slower, calmer tarantula species are easier to work with. I HIGHLY recommend using tongs whenever doing any type of maintenence in the enclosure, you'd be surprised how fast they can be when they think something is food LOL.
For scorpions, there are some quick little guys but most are laid back and won't shoot out of nowhere in order to get to your fingers. I can easily do spot cleaning and water changing without tongs for my Emp Scorpion because he pretty much sleeps all day long. But feeding them with tongs is also highly recommended.
As where their native to effects how they should be kept. Some require higher humidity than others. For higher humidity it may take a little more effort in tanks/display cages/kritter keepers but with tubs it makes raising the humidity a lot easier.
All in all, both inverts are pretty simple to keep. Scorpions are definitely extremely fun pets to have, and handle!