Raspberry Pi has been the standard for single board computers. The Pi has the largest community of users, the most project guides written, and the most tested software packages for a cheap computer that fits in the palm of your hand.
The new Model A+ occupies a frankly strange place in the current line-up of Raspberry Pis. The A+ was originally the smallest, cheapest Raspberry Pi. It’s since been eclipsed by the Pi Zero and Zero W, which are even smaller and cheaper.
What the A+ offers is the same processor as its fuller-featured big brother, the B+. It clocks at a respectable 1.4GHz, with four cores. That’s a hefty bit of computing power for a machine that can hide under a credit card. It also adds the wireless networking and Bluetooth capabilities found on the B+. Then it gets a cheaper price tag and smaller size by trimming features that not everyone will need, pruning off the ethernet jack, a bunch of USB ports, and half the memory from that same B+.
With that, the A+ may just be the Goldilocks computer for specific jobs, not too big, not too small. Pick the A+ when you need the full computing speed of the B+, if the weaker processor of the Zero won’t cut it. Or when you’re going to build a single-function device that never again needs a keyboard and mouse after you’re done setting it up, because the cost & space savings of the A+ vanish if it’s tying up a USB hub.
If you want to leave a keyboard and mouse plugged into your gizmo, the wealth of USB ports on the larger B+ make it the better choice. If you want a tinier or cheaper “full computer”, look at the Pi Zero W. But if you’re making a media server, or a cluster of devices that are managed over the network? Then the Model A+ could be the perfect fit.