USB is much faster, and just as reliable as a MIDI cable. The 'problems' with MIDI-over-USB are mostly caused by cheap and nasty interfaces (for example the very cheap ones you can find on Ebay) not by the technology itself. I wouldn't be concerned about whether a reasonably good quality keyboard had MIDI cable or USB connectors - though if it only has MIDI you will need a MIDI-to-USB adapter to connect it to a PC. They used to have MIDI adapters that worked on the joystick port, but now almost everything is connected by wireless or USB. PCs have never had genuine MIDI ports built in.
