That's a tough one... if I choose T-rex, do I live in the cretaceous, or in the present ? do I roam free, or am I encaged ? am I fed goats, or lawyers ?
After a night of fitful dreams, Gregor Samsa awoke in his bed transformed entirely into a gigantic Tyrannosaurus rex. His sister had him in a zoo or something by noon.
Oxygen levels wouldn't be a factor. The "need high oxygen levels for large size" only applies to invertebrates with very simple breathing systems.
If trex had similar type of lungs as modern birds, they would do just fine. Bird lungs are super efficient (much better than mammals' puny blind sack lungs) that enable them to fly at high altitudes.
So assuming I keep the same intelligence, totally the T-Rex. I'd be a gentle wonder paraded around the world wearing old-timey tailored suits, devouring the finest food, a fixture at every major world event.
It'd be the 2036 Olympic jetpack competitions and the camera would pan over, "an amazing day for the sport Jim, and look at that there's Terry the T-Rex sucking down chili dogs with his gorgeous partner, and the president of Earth 2" Yeah, that'd be the life alright.
A t-rex that is the only one of its kind and will be treated as a commodity like King Kong and will definitely be killed by humans, or a spider who can mate and live life and be ignored by humans? Hmmm, I wonder.
T rex just for the life expectancy. I wouldn't appreciate the media attention though, the tarantula would absolutely go unnoticed... Although... Thinking about it... Maybe Trex gets shot immediately.
I need more context information to give a proper answer.