I definitely would drill for any gates at the top of a staircase. If your DS runs into/falls onto a pressure-mounted gate, he could go tumbling down the stairs. VERY SCARY.
For the top of the stairs, I got Kidco's Safeway. Easy to drill the holes into the wall and rather than drill into the staircase banister, we got a 2x4 and bungee corded it to the wall. Kidco sells a kit that's MUCH flimsier than a 2x4 so I knew it was safe. Plus, I couldn't get that thing to budge at all despite using all 150 pounds of my strength pushing/leaning into it. Here's a photo of what we did.
Here's the side that had to drill into the wall:
I bought this one for the bottom of the stairs:
It's the Kidco Center Gateway.