MTA, easily because you can choose to not fukk with the USPS or deal with its infrastructure, which is all over the place in the case of the MTA.
MTA doesn’t have money issues like the USPS and yet still is on a 100 year losing streak with no end in sight. Also, the USPS isn’t cockblocking more efficient replacements. MTA on the other hand can’t just step aside and let the fukkin Hyperloop take over the bones of its infrastructure and put better shyt in place like an email did with day to day paper mail.