The short version
Both $99 budget. Bob and Brad Q2 Mini is the PT-brand mini portable. Renpho R3 is the Amazon-bestseller full-size with higher specs.
When Bob and Brad Q2 Mini is the right pick
If you want PT-brand credibility from "Famous Physical Therapists" YouTube and mini portable form factor — Bob and Brad is the right shape at $99.
When Renpho R3 is the right pick
If you want higher stall force + deeper amplitude in full-size budget form — Renpho is the right shape. Same $99; raw-spec value wins over brand framing.