its different, a lot...
Riveria skill is allowing her to have 3 level each for her 3 type of magic, for a total of 9 different magic in her disposal, hence the name Nine Hell (9 spell is actually a lot, coz the one who can be said as 2nd best mage on Orario from Freya familia, only have 3 spell on his disposal)
Lefiya's Double Cannon make her able to "save" a spell, alowing her to use it anytime she want... so she can chant the spell beforehand and just need to say "cannon" to unleash it later...
actually in term of potential, Lefiya can surpass Riveria easily... she have those "reserve magic" skill that actually really useful in battle as long as she can read the situation to "save" the right spell to use at the right time... and she also can use any magic from any other elf, which meants she can also use those 9 magic that Riveria have (i dont think she can do it currently tho)