You can use either filter in the under sink system. The IR-70E reduces more contaminants and is the same brand as your current IR-10A filter. It filters more contaminants than the IR-10A and the AP117.
http://www.waterfilters.net/Waterpik-IR-70E-Under-Sink-Water-Filter_p_97-379.html
Eventually, you'll want to replace your system since all parts and filters have been discontinued from Waterpik. If something on your system breaks, there are no replacement parts unfortunately. Here are all of our under sink systems:
http://www.waterfilters.net/Filters/undersink/undersink-systems.htm
Thank you,
WaterFilters.NET