Does the MOSFET Rds(on) value really matter?
The lower R_DS on in your proposed replacement will only help efficiency and won't cause any problems.

The output characteristic and transfer characteristic graphs are also worth comparing.  The TI part appears to need a slightly higher gate drive voltage to get "turned on" to the same degree at the ST part.  If the transistor were being driven with a marginal gate drive, that might be an issue, but from a quick look at the RAMPS schematic it appears the gate is being driven by a 5V output from the Arduino, which is enough to ensure that either of the transistors is fairly well saturated, so you should be good to go.