The 3 is a flagship phone (Snapdragon 800 series). The 3a is a mid-range phone (600 series) - less power, less speed. Also the 3 has the Visual core chip - the 3a doesn't, so camera processing is done in the CPU, which can take quite a while (seconds instead of milliseconds). If you're taking a series of HDR+ shots of Night Sight shots, it makes a big difference. (With the Visual Core, a Night Sight shot can take 10 seconds.)
The 3 is available in a 128GB version, if you need it. The 3 has wireless charging.
I have the 2 (with the 3 camera app port) - I'd say it is, if you're upgrading from something a bit older. (From the 2 + Camera 3 to the 3 isn't much of a difference.)