I would probably opt for the Series 4, unless you really want the ‘always on display/compass’ for the Series 5. If you’re looking for any sale prices, [depending on your region], Black Friday sales will probably offer some lucrative discounts on the Series 4 models, Series 5 discounts will probably be sparse.
For me, the "always on" face is important, and why I sold my 4 for the 5. But if that particular feature is not a deal breaker for you, and you can get the 4 for a good price, I'd say do it. It's an awesome watch.
Better bang for the buck? That would possibly be the S4, given when it was released and the amounts of retailers trying to clear stock of it.
Apple has now replaced the S4 with the S5, they kept the S3. And for me personally, AOD is not that big of deal.. granted I haven’t had any experiences with it. But a quick wrist raise... wakes up the display.