Which method grants the best xp for a duo-lane; staying in lane for last-hits while poking an enemy champion down, or running to another lane (Such as mid) to gank and try to get the champion kill for xp? Should I focus more on sustainability early game to get the extra minion experience or should I focus some glass-cannoning to try to pick up an early kill for the experience advantage? (Assume you have to return to base immediately after the kill, so you don't get minion xp afterwards)
The champs that are in lane should just focus on that first. If however, you pushed your lane to their turret and are waiting for the next wave to show up, you can look to other lanes to gank, not so much for the experience it grants but more for the gold your team gets and the gold you deny their champ during the laning phase.
For the second question, assuming you have warded for jungler ganks, you should quickly push minion wave to their tower to best give yourself the xp/gold advantage against your lane opponent. By the time both of you return to lane, the lane would have reset itself to the middle of the lane again.
The question of sustainability or kill lane depends highly on the lane matchup and who has the "stronger pick" for that lane. If you have the weaker lane, you should focus on sustain in the early game. If you have the stronger lane, then you can consider going for the kills, again not for the experience a champion gives but for the denial of experience/gold for the opponent champ after you can push the lane out.