Weezing offers you the ability levitate, meaning it is immune to ground. He also has 120 base defense versus Garb's 82 defense. He also can learn Will-O-Wisp to further sponge physical shots.
Garb has higher HP (base 80 over 65) and has access to Toxic spikes, which can have a higher payoff for you in the long run. Garb can also pick up spikes, and is used more for setting hazards as a lead or mid-game support.
For physical walling, I suggest going with Weezing for the job.