I just built a rig with an Athlon 3000G the other night. At the time that I purchased the parts, the 3400G was basically unavailable, and the 3200G was a little more than I wanted to spend. Plus, I was planning on overclocking the rig, and sticking in a GTX 1650 4GB D5 GPU. Which would make a fine Fortnite PC.
But I would size things (APU CPU-core-wise) according to CPU compute demands. The 3200G should be enough for most things, although the extra 4 threads of the 3400G can help with compute-bound tasks, and also some games prefer 4C/8T over 4C/4T, if you were to drop in a dGPU in the future. Because of memory-bandwidth limitations, and the fact that you can OC the iGPU, the 3400G with Vega 11 really doesn't offer than much over the 3200G with Vega 8, IMHO, at least for APU gaming. For watching 4K UHD VP9 vids (YT on a 4K UHD screen), either should be fine, just make SURE to use a dual-channel memory configuration.