Out of the two, Weezing is definitely the better tank of the two. That isn't saying much though, both of them are equally bad in this metagame.
Weezing gets a more than decent movepool for a staller though, in Will-O-Wisp and Clear Smog, Haze and Pain Split. It also gets Flamethrower and Sludge Bomb for more reliable damage.
As already said in the comments though, Mega Venusaur and Amoonguss are both much better walls/tanks, and actually do a good job. Other Pokemon include Tentacruel and Nidoqueen also do the job well. Some people even run bulky Roserades, that usually hide behind a Substitute and proceed to set up Spikes/Toxic Spikes and Leech Seed. Niche Pokemon like Defog Skuntank can also be pretty hard to take down at times.
Overall though, the best tanks among Poison types are almost undoubtedly Mega Venusaur, Amoonguss and Tentacruel.
Hope I helped!