@Taylorloveall we love helping with these kinds of questions, so thanks for reaching out!
The Nemo Forte 35 and Nemo Disco 30 are both great bags! Despite similar temperature ratings and weight, the most significant difference between the 2 bags will be how small they pack down - the down bag (Disco) will back down much smaller than the Forte, which will be quite important if you intend to use it for backpacking.
In terms of length, we'd definitely recommend the long length for you at 6'3. The regular length in these bags fit up to 72", which is a bit short for you and means you will likely not have enough space around your feet and toes to keep warm.
We hope this helps; let us know if you have additional questions!