Honestly: what I would prefer doesn't really matter.
Just a few notes:
With latest 4.14 kernel there is HW accelerated crypto possible with Exynos 5422: https://wiki.odroid.com/odroid-xu4/software/disk_encryption -- no idea how easy it is to use though and you should keep in mind that Armbian's next branch still uses kernel 4.9 (the last try to switch to 4.14 ended up in bricked boards but once this is resolved this functionality should be available with Armbian too)
RK3328 implements ARMv8 Crypto Extensions so at least software using AES will benefit from this automagically. @chwe pointed already to threads with performance numbers, you could also have a look in ODROID N1 thread to compare cryptsetup benchmark numbers (though made with RK3399 so you have to do the math on your own using RK3328/RK3399 openssl numbers to compare HC1 and Rock64 performance here)
since you mentioned 'german customs fees': buying at Pollin should ease warranty handling and stuff: https://www.pollin.de/search?query=odroid hc1&channel=pollin-de
Since you compare two boards running different architectures and since you seem to be keen on 'more DRAM' you should keep in mind that memory usage with a 64-bit userland can be much higher compared to running a 32-bit userland. See especially the links I reference here: https://www.raspberrypi.org/forums/viewtopic.php?f=91&t=192321#p1281326
In case the tasks you want to use are memory hungry running with an arm64 userland on Rock64 can result in lower performance compared to HC1 using the same amount of memory. So choosing an armhf userland instead might be an option. Not possible with Armbian right now but since the excellent ayufan images exist a good basis could be his armhf OMV release: https://github.com/ayufan-rock64/linux-build/releases (grab latest image marked as 'release' and avoid 'pre-release' if you're not an expert). AFAIR Nextcloud should be installed best in a Docker container then...
