Xiaomi Redmi 4 Prime (Banggood) Vs Redmi 3S Prime - Worth Getting?

So, in the recent 11/11 sales I bought the Redmi 3S Prime for USD129 - bargain! (I want a small phone that's still half-decent, and all there really is is the Redmis and the Xperia compacts. I prefer the smaller size of the compacts, but have had very bad experience with them - my Z3c after one year has a shattered back (from a knee-high slide off the table onto a flat smooth floor - it impacted on the corner of the phone and the back just shattered), the earphone port doesn't work, and the microUSB flap doesn't close properly. So disappointing, because the camera's amazing).

Then, rage-inducing - a mere 5 days later, the Redmi 4 Prime (not a Note) is available from banggood for USD137!
It seems better than the 3S prime in every way except being 10g heavier and 2mm taller (but the width matters more than the height when it comes to using the phone with one hand, I find). Notably, a 2.0GHz octa-core instead of 1.4GHz octa-core.

I'm very tempted to buy this phone as instead and then try to sell on the 3S Prime (which I can't cancel).
It seems like a great deal, which automatically makes me suspicious.
Can anyone think of a reason not to do this?

Is the 3S Prime going to be hard to sell now the 4P is out? (it'll be brand new, but only arriving … I wasn't expecting it to arrive this year, to be honest). Is the CPU upgrade noticeable? The bands should be OK for Aus. Is it going to have a Chinese ROM? (The 3SP page made a particular point of mentioning the international version of the ROM. Am I going to have to install the international version on this 4P? I've done the unlock bootloader/root runaround with 2 or 3 phones now, but it was quite confusing).

And finally, the 4P is a preorder - stock incoming 30 Nov. I don't mind waiting as it's only a ~2 week delay on my 11/11 3SP order, and as mentioned, I don't really expect it until early next year. But does anyone have experience with banggood preorders? Are they likely to be delayed? Do they ship promptly from the date stock is expected? etc.

Thanks so much for your input. It does seem a great deal, but I'm aware that I'm still grumpy about ordering the 3SP just 5 days ago to have a seemingly-better phone appear for pittance more, so I just want to make sure my head prevails over my emotions..