Portability and Weight
Weight is an obvious consideration, especially when you'll be regularly hauling the PA System to the different places. As such, you want one that's as light as possible without compromising sound quality and the coverage that you need. Having sturdy yet comfortable handles are also a important. You may also consider adding wheels, or using carts to relieve some of the carrying. Compact form factor is also important, you want one that can fit into your mode of transportation, especially when you'll be carrying other gear like instruments, where every inch of space you can save is important. Suitcase style PA systems score high points here, with their space-saving design. Another often ignored but important feature to look out for are storage compartments, so you won't have to carry cables separately.
Power and Coverage
Portable PA systems are meant for small venues (with some exceptions), so don't expect them to be loud enough for big stages or open air concerts. Still, you want one with enough headroom to get your sound to your audience as clearly as possible. The PAs featured here are all capable of handling small venues, some can even cover medium size ones. Note that the higher the output power, the better the coverage, but at the cost of extra bulk and weight - so you'll have to carefully weigh your needs to get the system that is at just the right size and power.
The portable PA systems that made it to our list have two or more channels available, and multiple input options. Some even have Bluetooth connectivity for convenient wireless streaming. Again, you'll have to assess the input ports that you need, and get one that can provide you with enough ports, or maybe go for one with more, for future expansion. On the other hand, you can get a standalone mixer to expand your input options without having to go for bigger PA systems.