Lightning strike surges average about 30 kA (yes, kiloamperes), peak at roughly 100 kA, and follow a roughly triangular-exponential waveform with a 10 µs rise and a 1000 µs fall time. However, the peak voltage is only around 1.5 kV due to the low dynamic impedance offered by the ionized arc core. This is still enough to pose three hazards, not just two, though:
The classical hazard people think of when they think of a lightning strike is electric shock — and a 1.5 kV source can definitely do that; however, the short duration of the impulse means that it’s the equivalent of a single big zap as opposed to a sustained contact, which likely would reduce the risk of the heart going haywire (fibrillation), ironically. Furthermore, being inside a metal thing definitely has something of a Faraday-cage effect, even if it is imperfect. Short-distance arcs have a resistance on the order of tens of ohms, so even a couple of gaps in a suit of fullplate will not keep it from diverting the bulk (>90%) of the current around the body instead of through it.
The high currents involved in a lightning strike cause significant resistance heating to anything that tries to conduct them; this is a major damage mode for wires and other metallic objects that must endure a strike, and is the reason strike terminations (lightning rods) use hefty, dedicated grounding conductors. Fullplate is 5–10 times thinner than a strike terminator, and is made of a poorer-conducting material (steel). Still, the sheet resistivity of steel is on the order of 10−4 ohms per square, which means that a breastplate offers very little resistance to electricity overall — but either way, the high currents involved yield a peak power dissipation well into the megawatt range, more than enough for significant Joule heating to take place.
I’d expect further heat contribution from induction heating, however, this effect is much harder to compute as it’s strongly frequency dependent.
This is the least-known mode of injury involved in most electrical incidents, but it causes the bulk of the actual damage, and can seriously or fatally wound a victim without shocking them.
(See this later answer of mine for more in-depth discussion on arc flashes in a similar situation.)
Arc Flashes consists of a combination of intense heat (temperatures of ~20,000 Kelvins), radiant energy (from far IR to deep UV), concussive blast caused by the vaporization and rapid expansion (tens of thousands of times) of metal, and rapidly propelled superheated shrapnel (a result of the other factors); taken together, an unmitigated arc flash is easily equivalent to nearly a pound of TNT going off in your face, if not more. This would be the primary damage mode the knight would have to contend with, and could easily destroy significant portions of the knight's armor and gambeson instantaneously, while leaving him contending with the equivalent of a small pipe bomb going off at point blank range.
End of story: the knight is a lot more screwed than basic electrical knowledge predicts. Expect him to be laying on his back with a gaping, smoldering hole in his breastplate, wondering WTF just hit him.