Okay. Did some more rolls.
All in HC mode, all 8v8's, all with races that give no attack or defense bonuses (including the blademasters I've faced, due to no boosts being used). The attacking is higher due to me toying with some victims, oops!
Keep in mind these sample sizes are very small and against races with NO bonuses active (e.g. elves, arachnids - being attacked, haven't been attacked by one yet, blademasters - no boosts, natives, and undead).
Controls I have when hosting a game:
boosts off
upgrades active
borders
HC
Attacking (win%) n=46
~56.5%
Defending (win%) n=31
~61.3%
I have yet to try anything in attrition, due to a personal dislike of it, but I will get around to it in due time. Currently I have my excel sheet sorted by race, then by win (yes/no), then by mode (which means attrition will factor in eventually).
Just in case anybody was curious

Edited for verb case, oops!