How to generate a high energy electromagnetic wave? Using high voltage or high current?
If you are generating an electromagnetic wave in free space then the ratio of the E field to the H field is fixed at 377\$\Omega\$ (yes, ohms -- the units work out).  

You would hope that would answer your question, but it doesn't because in order to generate a pulse or continuous waveform of electricity and turn it into an electromagnetic wave you need to couple from your circuit to free space (or to whatever material you want the electromagnetic wave to be in).  The device that does this is an antenna, and depending on their design antennas can have very different input impedances and efficiencies, even though the EM field that they generate has a ratio of E to M of 377\$\Omega\$ in free space, or the characteristic impedance of whatever material you're launching the EM pulse into.

So the actual answer to your question is "either one, depending on your antenna".

(Note: I looked at whether I should vote to close your question.  I don't think I should, even though the answer to it as written is "this question is unanswerable".  To really answer your question requires about as much information as you'd get if you got a Master's degree in RF circuit design with a lot of attention paid to antenna design.)