Reading through the SoP Wild Mage, it seems like a flavorful slight downgrade to the base Thaumaturge.

You lose some useful invocations like Defensive Invocation and Empowered Attack/Defense, but in exchange you get to keep Wildfire up all the time as a buff rather than Soulfire sometimes as a neat bonus. You lose the opportunity to double-dip your forbidden lore bonus, but in general, that feels like a wash.

The one big change is exchanging Empowered Resistance at 15 for Defensive Maelstrom at 3. Sometimes-on 100% miss chance doesn't quite compare to always-on mettle. Even if it does fare better vs. rays, it does nothing vs. battlefield control. Even if it is a downgrade at level 15+, with Defensive Maelstrom coming in at level 3, I expect this to mostly come down to what levels you're playing at.