Hey there. I actually love both readers and I read with them often.
I'd say I use Mattie much more sparingly--her visions are pretty stellar. She is one of those true-born psychic visionaries that we've heard about since the Middle Ages. Women like here were either revered as saints or burned at the stake for being a witch lol.. She's that level. For example, my first call with her (I've only had three) she knew why I was calling. She got insanely uncanny details like names, places, and reasons why. I'm still waiting for her major predictions to manifest later next year. Some things she throws out may be non sequiturs at the moment, but they also may manifest.
Effie uses the tarot. While Mattie may reference something like "Why am I being shown the Queen of Cups? Spirit is showing me this," Effie actually pulls a tarot spread for you. She has been pretty much 100% accurate with her current situation spreads. Her major prediction for this month doesn't seem likely to happen, however, due to extenuating circumstances. I'm bummed about that but am reading with her on Monday to clear things up. She has been accurate with smaller timeframes, however, and is totally on-point always with my present when I can't understand things clearly.
I hope this helps. Each has their own merits. But, like I said, Mattie is a born visionary. It just depends on what kind of read you're going for.