Hi katelyng,
Welcome to the Mattress Forum! 
The best place to start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you make the best possible choice.
There is also more about the new “breed” of mattresses that only offer a single firmness choice (including Tuft & Needle and Casper) in post #3 here that can help you compare them.
The Casper has 1.5" of synthetic latex as part of the comfort layer but the upper layers of a mattress are primarily for comfort and pressure relief and “feel” not for support which primarily comes from the firmer polyfoam support core in their mattress.
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here that can help you make more meaningful quality/value comparisons between mattresses.
As Nynygirl77 mentioned 3 posts earlier … I would also keep in mind that for the $250 difference between them you could purchase a 2" latex topper on top of the Tuft & Needle as well.
I would keep in mind that there is no “standard” definition or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that is firm for one person may feel like “medium” for someone else or even “soft” for someone else depending on their body type, sleeping style, physiology, what they are used to, and their individual sensitivity and perceptions. This is all relative to each person’s perceptions and the only way to know how firm or soft a mattress feels to you (regardless of how firm or soft it may feel for someone else) will be based on your own personal testing or sleeping experience.
Phoenix