Hi Brandon, interesting question, which anybody should only answer with "that depends".
I'm not into those API's. But 10 yrs of being responsible for defining and offering solutions like this have made that even harder to me. Because IFA's tend to have very different understandings what they need for their job and what a platform needs to do. So you would need to take a detailed look into functional (e.g. wanna have KPI like TWR, MWR or VaR for the individual customer transactions? - Well, that is something most of the API's don't offer) and technical issues (what kind of multi-tennant capability you will need).
And in addition to the already named providers of financial technology - there are a couple more offerings out there. For example aixigo the company I'm proud to represent.
Please also consider that API's can differ in many ways. Look out for restful API's (for easy integration) from "experienced" providers.