Since they are the only 2 guards below 26 y/o in this conversation, here are some of their defensive stats from the last season. Both of their clubs were pretty poor defensive teams (28nd & 22th)
Iso defense
Booker: 1,13 PPP (8,4% percentile)
Kyrie: 1,08 PPP (12,3%)
PNR BH
Booker: 0,81 PPP (62,4%)
Kyrie: 0,96 PPP (18,7%)
Spot up
Booker: 1,07 PPP (31,5%)
Kyrie: 1,14 PPP (18,3%)
Spot up
Booker: 1,07 PPP (32,7%)
Kyrie: 1,14 PPP (18,3%)
Handoff
Booker: 0,85 PPP (18,7%)
Kyrie: 1,14 PPP (14,4%)
The stats suggest that both are below avg. defenders, but Booker showed some promise defending a guard that is creating with a big (PNR BH 62,4% percentile, Handoff 57,2%) while Kyrie truly dies on these plays involving screens (PNR BH 18,7%, Handoff 14,4%)
Booker is showing a bit more upside on the, for both players critically flawed, defensive end. It surprised me a bit.