There's certainly tradeoffs, but a couple of additional points to consider:
From a processor perspective, the 670 should be very comparable to the S8+'s 835, so all other things being equal, performance should be about the same - except that they're not equal, with Google likely to perform slightly better due to the wholly integrated design of the hardware/software package.
While the S8+ saw Pie, it's questionable whether it will receive Q, and extremely unlikely it will receive R or S (if you were to keep it that long) or timely monthly security updates for an extended period; the 3a XL would remain relevant in terms of software for at least 3 years.
I assume you have a memory card and, much like having an extra closet, any space you are afforded you are likely to find a use for. The question therefore becomes, how much storage do you actually need? If you generally use that card to keep a library of media, for example, can you keep a sufficient store directly on the device to comfortably accommodate your needs; if so, the expandable storage is a luxury to which you need to assign a monetary value.
Lastly - and this would be one of the largest considerations for me in your shoes - your S8+ is (presumably) out of warranty, meaning you can get great value out of it now if working fine. But if something fails on, it you have a large opportunity loss in terms of potential trade-in/sale value, whereas the 3a XL would carry a new warranty.