Industrial automation guy here.
Mechanical relays and contactors are not designed for rapid cycles. During operation there is always a small voltage jump as the internal contacts near each other while closing. This builds up heat and carbon over time (how fast depends on voltage, amperage and...
Sorry to resurrect an old(ish) thread. Just wanted to add that the AB Micro800 series not only has decent software but it's free as well. They are also fairly cheap as far as PLC's go. I certainly recommend looking into that series if that's the direction you're aiming to go.