I don't think the things that make Gardiner better than Rielly on the PP are shown with statistics. PP goal against the Rangers. Unassisted end-to-end goal where he fools every ranger on the ice. They have no idea he's going for it until it's too late, to the point that it looks almost ridiculous that nobody just poke-checks him. How does he do it? Because he's looking at his obvious pass option the whole time, and he's a deceptive skater. Lulled them into thinking he was slowing down when he was actually maintaining a high enough speed to split the D, by leaning the wrong way. Now Rielly is a great skater, but he's not a deceptive skater. He can go end-to-end but he doesn't make it happen out of nowhere, and he doesn't have the kind of wrister that Gardiner used to beat Lundqvist there.
I like Rielly more than ever, but I'm not convinced he has the same offensive instincts as Gardiner. Gardiner does some really creative stuff from the point. Look at his primary assists on the Nylander goal against the Jets. Not a PP goal, but a play from the point where he just straight up fools everybody on the ice. Nobody knows he's going to Nylander, not even Nylander. In fact his body language is so convincing he almost loses his balance and falls over passing it back across his body.
His assist on the Bozak goal against the Rangers. Again, not a PP goal but a play where he starts from the blue-line and fools everybody. He's got 3 Rangers mesmerized thinking he's going to do a lap behind the net. If Rielly were making these plays there would be no question who our long-term #1 D-man is going to be. But I feel like Rielly gets tunnel-vision with the puck, where he's decided what he's going to try rather than reacting to the situation. I have no other explanation why he'll so often rush the puck only to force a pass that isn't there, or do the opposite and miss an open lane. He holds onto the puck too long.
Compared to Gardiner you get fewer mistakes but also fewer single-handed offensive contributions.