If you wanted to flash OpenWrt on one of these decade-old routers to use as a 5GHz AP + managed switch, maybe with a USB drive connected for an NFS media share, which one would you choose, and why?
This would be used in a low-demand, negligible-interference situation for the WiFi, and handle at most 4 VLANs for some nearby wired devices. The 1 Gbps backhaul will be perfectly sufficient for this purpose.
Please stick to answers choosing one of the two options. I'm well aware that buying something newer would be ideal; the point here is to make do with what's already available. Unless there's some massive, known, unmitigated hardware security flaw in both devices, of course, which would also be a valid answer/warning.
I'd choose the Asus because it's Qualcomm, which is slightly less FOSS-hostile than the Marvell in the Linksys. It might have a longer life due to that. Both are good enough (RAM- and flash-wise) to last another 10 years, assuming that the driver updates and/or binary blobs addressing any security issues are forthcoming from the respective SoC makers.
The wrt1900acv1 has the faster CPU, more RAM and is able to route at faster speeds (close to 1 GBit/s), but its WLAN side is very, very temperamental (read that as bad and not recommended). If WLAN is -in any capacity- part of your usage scenario, don't go with this.
The RT-ACHR17 has four slower cores (compared to 2), maxes out around 300-250 MBit/s routing throughput, has less RAM, but good WLAN cards (yes, they're worse on paper, 2x2 vs 4x4, but they actually just work™).
Neither of them are dinosaurs, yes they're wifi 5 and not 6/ 6e/ 7, but that still useful for many scenarios, but if WLAN is on the requirements, there can only be one answer here.