I would say the 250 mA (a load rated is in mA not mAh) load should be OK for both batteries. Just attach the loaded voltmeter, give the battery 10 seconds or so to stabilize and take a voltage reading. If the voltage is still dropping after 10 seconds wait longer until it stabilizes then take a reading.

As far as what volt meter to get, I use a general purpose digital multimeter with an external load, so I am not of much help with regard to RC specific loaded voltmeters.