I all depends on several things,
First both are good, Kato being a little more costly but has roadbed attached and wiring for turnouts will be simplified with the Kato's plug in system and all equipment will operate on it.
The drawback is that Kato only offers fixed radius curves.
Atlas offers the widest variety between the two and both a code-80 and 55.
Wiring will be more complex and the code 55 does not allow the running of equipment that has large flange wheels, that is you will need equipment that has NMRA code compliant RP-25 wheels and most of the new equipment today does.
The atlas code 80 allows the use of all equipment.
Hope that this helps.