Using a DC Electronic Load vs power resistors (dummy load)
The spikes you are seeing should not be due to using a resistive load.  Probing technique is critical in looking at output ripple in a power supply.  Usually you want to get the scope probe right across the output terminals of the supply, or across the output caps.  You can't use a long ground clip either.  I usually wind a spring around the bare barrel of the probe (GND) and solder the spring to the negative output with as small a pigtail as possible.  Then I do the same on the tip.  Also, ripple is usually measured with the 20MHz filter on the scope active.