1st off a mantra is sanskrit for a chant. Sanskrit mantras work based on sound frequency to vibrate a certain way and cause a a desired outcome with little meditation necessary.
2ndly a spell is a written array of words (can be any language) put together sometimes in prayer form so it may be meditated on and cause a change in the physical world.
So what's the biggie? Does it really matter what method you use? Most new age magicians say do what works for you. I say try something new!
When I was younger I would scavenge the Internet for spells that work but there was either stuff I couldn't get for the spell, or they were fake or just not what I needed really. I didn't even know how important meditation on the spell was!!!
To make it worse, supposed high experienced mages don't like helping!!! Some newbies wanted help and we're made fun of including me :/
So what this got to do with mantras? Easy, you don't need spells. Why? Because mantras are basically spells but without tons of meditation or ritual. Mantras were made by ancient sages and mages who practiced them and have been used for 1000s