Peak = a "peak" in the frequency response. A certain frequency (or group of frequencies) will be much louder than the frequencies above and below them:
See the gray, purple and red lines? They have "peaks" in the frequency response. They are good for obtaining more "peak" output, but bad for smooth response (optimally smooth [flat] response would be the black line).
LFE = low frequency extension. How much output the subwoofer has as the frequency decreases.
Larger enclosure = lower Qtc, which means flatter frequency response, better "transient" response, better low frequency extension, less overall output and better efficiency.
Smaller enclosure = higher Qtc, which means peaky frequency response, worsened transient response, worsened low frequency extension, more overall output and worse efficiency.
Just need to decide which box better fits your tastes. Smaller box is more for obtaining higher output, larger box is aimed more for sound quality type installs or where power is limited.