PandExo was written using the core capabilities of STScI's Pandeia (Pandeia + Exoplanets = PandExo). The front end of Pandexo is designed specifically for exoplanet transit studies. You might ask-- what's the difference between running Pandeia and PandExo? Pandeia requires that you know the number of groups, frames, and integrations. It also only takes in as input a stellar SED. PandExo takes in a stellar SED, a planet spectrum, and the transit duration and provides, as output, the error obtained on your spectrum. You also have the option of optimizing the number of groups, per integration or hard coding your own. PandExo has nice front end plotting functions built in, as well as parallelized bash scripts for running multiple noise models at once. Fundamentally, they also use different methods for computing noise. For a complete explanation of this, see arXiv:1702.01820
A detailed description of PandExo is available in Batalha et al. 2016. For specifics about the code please visit the PandExo's Github.io.