In short you're asking about two different SoCs and radios.
WRT3200ACM uses a Marvell Armada 385 SoC which is less powerful than the IPQ8065 in the R7800 but not by a huge margin. I honestly doubt you'll notice a difference in the end. What you should have in mind is that Marvell SoC(s) are by quite a bit more popular in the open source community and therefore have more mature support (including upstream). While there has been quite a bit of work put into the IPQ****-platform lately it's not as mature however that doesn't necessarily mean that you'll notice the shortcomings in terms of platform support. I do know however that there has been (not sure if it's been resolved yet) issues with ethernet on the IPQ806* which you might want to look into.
In terms of radio performance it's a mixed bag, I have a few WRT3200ACMs and they all work great without any issues at all however as with everything else some are running into issues. Keep in mind that in my case I can only vouch for AP mode, WDS supposedly works too now (quite nifty in some cases but might not be something that you'll ever use) but it's not something I've tested.
ath10k which is the driver that IPQ8***-devices normally uses for radios has been around for a longer period of time compared to the mwlwifi driver however quite a bit seems to be very chipset specific meaning that some runs a lot better than others despite being the same driver. In all fairness I don't have any specific experience with QCA9984 which the R7800 or the Zyxel uses however I do have with QCA9986 which is a cut down version and I can say for sure that it performs much worse than the mwlwifi driver and crashes easily. That said, it may boil down to limited memory (it's a TP-Link C59 with 128Mb of ram) as ath10k seems to be a bit memory hungry. Neither are and/or will ever be rock solid in all kinds of environments but given the overall feedback I'd say they're equally good or bad depending on your viewpoint.
Regarding flash, you'll have a hard time pushing above 32mbyte so flash size isn't really going to be much of an issue irregardless of what you go for. I'm sure someone is going to object but you have to consider the compatibility of the hardware itself and what you're going to use it for. As a router running OpenWrt even if you add a lot of network related software you'll probably end up with an image below 16mbyte, if you want use external HDDs bump that to ~30mbyte-ish if you want both Samba4, Minidlna (ReadyDLNA) with a recent version of ffmpeg, filesystem and partition utilities. Your limiting factor is however by far the amount of RAM, 512Mb isn't huge if you start to run services and you can pretty much forget about running IDS etc on that amount which limits the selection of software and therefore also the needed amount of storage. Also, if you want to attach storage, you'd want to use eSATA instead of USB3 for sure so keep that in mind.
If you really want IDS, anti-virus scanning etc x86 is pretty much the way to go and OpenWrt might not be the best software for accomplishing that. Getting a dual setup as dlakelan suggested is most likely the best route for less headaches. However you probably want to get something newer than J1***-series CPUs but that's another story....
In short, both will probably be fine however I would personally lean towards the WRT3200ACM because it has more mature support even though it's a bit slower.
However, given that you seem to live in US/NA I'd say that you should give the Espressobin + a MT7621 wifi module a consideration as you get a very good bang for the buck ratio and quite a bit more ram too which may be worth it depending on what you want to use it for.
Keep in mind that MT7612E only supports 5Ghz so you could potentially use your old router as a simple AP for 2.4Ghz if needed at all (I don't run 2.4Ghz at all just fine). It's slower in theory (linkspeed) however the driver is pretty much open source and is being actively development so in theory that should be the best option in the long run in terms of support. Regarding overall wifi speed very few clients have more than 2 antennas so link speed is going to be below what 4x4 provides and while there are some other advantages they are more or less negligible in home user environments.