I Have Done Both. When the PAX airline was financially healthy life was good.
When things got bad financially things were not so good.

FDX has remained financially healthy for most of my career (knock on wood) as Past performance is No guarantee of future reward.

Both PAX and Cargo have their +'s & -s.

There are a lot of fly by night (excuse the pun) Freight outfits. Many of which aren't worth your time as they are usually temporary jobs at best and fly older often unsafe equipment.
I would avoid almost all of them unless you need the time experience as a stepping stone.

Unless you get lucky and get hired by FedEx or UPS, I wouldn't go the Cargo route, especially if you can get on with Delta, SWA , ALA or even UCAL.

If you do get an offer from FDX or UPS and maybe ATLAS it can be a solid career. (We have hired many Atlas pilots at FedEx)

The Biggest Negative to FDX or UPS is the Backside of Clock flying which you will most likely do as a Junior pilot on the list.

The Biggest Plus is more Stability (notice I didn't say guaranteed stability) and current Pay/benefits but that can change overnight (again excuse pun).
Look at UPS, they have Pilots on Furlough and FDX came close a couple years back, so no one is immune to down turns in the economy, even when the company is still profitable.

No passengers and Flight attendants to deal with. There is less hassle with Cargo in that department. When the plane is loaded, we leave. We don't wait for scheduled departure time if we are loaded and ready. With FDX or UPS you will get a Chance to see the world, often in one 14 day trip around the world.

No Pass/travel benefits on Cargo although those benefits from the PAX carriers aren't what they use to be, but I still miss them.

UPS and FDX have fewer Domiciles to pick from. Unless you are dying to live in Louisville, Memphis or Anchorage you have more to choose from with the PAX. I hope you like New York, Atlanta , Detroit , LA, San Francisco, Dallas or Washington DC.

There are other considerations but those are some biggies.

Bottom line and you will hear this a lot. Get an offer first and if you get lucky and have two or three offers, we can have a deeper discussion at that time.

Good luck