ESP8266 eLUA (NodeMCU) vs MicroPython
Here's a slightly different approach in stead of a Lua vs. Python shootout:

Six of the most popular ESP8266 "runtimes":


AT Command SET. Popular when the 8266 is paired with another MCU.
Communicates via the serial port. ~64k of 128k RAM available. 
MicroPython.  A MicroPython script interpreter
    with user friendly GUI that can be accessed via the serial
    port or WIFI/IP. ~30k of 128k RAM available.
Lua/NodeMCU.  A LUA script interpreter with user friendly GUI that
can be accessed via the serial port. ~40k of 128k RAM available.
JavaScript/Espruino. A JavaScript interpreter with user friendly
GUI that can be acess via the serial port or WiFi/IP. ~20k of 128k RAM available. 
C/IDE-12E. ESP8266 flashing tool &amp; C libraries/tools using the standard Arduino IDE. ~80k of 128k RAM available.
C/ESP8266_SDK. C libraries/tools from the manufacturer. Also a collection
of example applications. ~512k Flash. Guesstimate 80k of 128k
RAM available.


The key insight is that the bulk of the code is common. The primary libraries in 1-5 all originate from 6. Beneath a thin layer of AT/Python/LUA/JavaScript/C the primary code is practically identical. That means performance (RAM, FLASH, execution) is similar too.

Since you seem concerned about speed and RAM (flash is generally OK) how about option #5? Arduino is a usable IDE with a large collection of examples. You could have your first code running in less than an hour and would likely outperform any of the  scripting engines.

In the absence of significant memory usage differences I would choose MicroPython due to the greater number of libraries and an active online community with IRC webchat. Documentation for adding C modules has improved.

Option #6 provides you with the highest potential for optimization but at  higher complexity and steeper learning curve.

Lastly a good ESP8266 rule-of-thumb: Each TCP/IP connection can consume up to ~3k of memory. Always expect less than 5 simultaneous connections!

TL;DR: ESP8266 applications have most of their code in common and perform similarly. So choose the script engine you like or step up to C/IDE-12E. Don't expect more than 5 simultaneous IP connections.