One thing for sure... higher chance of custom rom for SD660
Both are base on Cortex anyway. Different is SD release source code and driver. So is easier to have custom rom develop. Even for the manufacture's own developer team, they find this helps in developing new function. This is why even for Xiaomi, you see different in product update for SD vs MTK.
Kirin SoC face the same issue.
Kirin is Huawei own code, mix and match Cortex and GPU on SoC. If Huawei don't release fixes. No one can (just like iOS).
Anyway one is eMUI and one is MiUI. They are base on same open platform.