The T-shirts let the Venom design do its thing without much else going on around it. That suits a universe where teeth, tongue and black symbiote already take up more than enough space.
Posters give the design more room and work better when Venom is headed for the wall.
No need to overcomplicate things. Pick clothes if Venom is coming out with you. Pick a poster if he’d rather stay behind and keep an eye on the room. The symbiote is just one of many heroes and villains in Marvel.