In terms of the difficulty of the questions, the PiCAT and the ASVAB are the same. However, the PiCAT could be considered easier because it is taken without time pressure, but you will have to sit a verification test to get the score used in your military application.
Remember that 5% of PiCAT takers are randomly selected to take the ASVAB anyway, and if you fail the PiCAT verification test you will automatically have to take the ASVAB.