Kevin,
Are you sure that the Neo-M8P does not log data? The product summary - https://www.u-blox.com/sites/default/files/NEO-M8P_ProductSummary_(UBX-15015836).pdf seems to say that data logging is possible.
From you post comparing the SmartNav and the Tiny RTK
TinyRTK :
easy integration
serial connectivity
easy handling (you only have access to restricted number of parameters)
no raw data logging on-board, no post processing with u-blox algorithms
extremely small, low energy
fully compatible with Pixhawk
time to fix : approximately 4-5 mins in good conditions (u-blox has just released a new firmware improving these capabilities so it should be even better now)
integrity functions included (a little less precision, but better reliability as-is)
I have probably forgotten some features so feel free to ask any specific question about the systems.