the Pi 3 range is a sweet spot for low cost, low power draw, decent-enough CPU. Newer models draw increasingly more power; going from 1.4W to 2.8W may not seem like much, but that’s half your battery life. There’s a few differences in Raspberry Pi 3 versions that may lead you to buy one or the other:
The Pi 3B has 10/100 Ethernet, 802.11n (single-band) WiFi, Bluetooth 4.1. Power idled at 1.4W and peaked at 3.7W.
The Pi 3B+ removed the 10/100 Ethernet in favor of USB Ethernet (~300Mbps w/USB2.0). CPU cores were overclocked from 1.2GHz to 1.4GHz (so a heatsink is more necessary), with ~15% increase in benchmark performance. It added 802.11ac (dual band) WiFi and Bluetooth 4.2 w/BLE. Power idled at 1.9W and peaked at 5.1W. This is also the only 3-model supporting PoE (w/ extra HAT).
The Pi 3A+ removed Ethernet and reduced USB to a single port. The RAM was reduced from 1GB to 512MB. Power idled at 1.13W and peaked at 4.1W. The A+ form factor is more compact. Overall the 3A+ is smaller, cheaper, and less power draw than the 3B+ (but not as low as the 3B).
The lowest power draw with acceptable performance is the 3B. For slightly more power draw and more CPU performance, go with 3A+. For “everything” (including PoE) the 3B+ is it.
If you want the 3A+ but don’t need the video, want a smaller form factor, and half the power draw, the Pi Zero 2 W is it. Though the Pi Zero 2 W is supposed to be cheapest, due to demand it’s often sold out or more expensive. The 3A+ is still cheap (~$25) and available, with the downside of the higher power draw and larger form factor.
(disabling HDMI, LEDs, Wifi, Bluetooth, etc reduces power draw more. in testing, the 3A+ drew less power than the Zero 2 W with everything disabled. all of them draw ~0.1W when powered off)