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
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.