24VAC vs 220VAC inductive voltage transient
7 VA at 24 V AC means a current of 7/24 amps or 292 mA. From this, the impedance is 24/0.292 = 82.3 ohms. From the cos(phi) bit you can then calculate the series inductive reactance and resistance that add up to the 82.3 ohms.

So, assuming you can do this (I get XL = 78.5 ohms), convert this to inductance and you have everything needed to compute the stored energy (\$\dfrac{I^2L}{2}\$).

Compare the energy in both scenarios and the one with the highest energy storage capability is the one that could generate the bigger spark.


  What's the comparison between both voltage spikes when the coil is
  suddenly opened ? (How much higher, how much longer, frequency, etc)


Higher and longer is about energy and control of that discharge of energy. Frequency is much harder to predict because you haven't stated what the parasitic components (or sunubber) are.