Absent other context, both sentences are perfectly acceptable and virtually interchangeable. However, they will change meaning substantially if you provide additional context.
An imperfect sentence generally has two interpretations. Either it represents something on going, or something habitual. In either case, it is something that occurs simultaneously with something else, hence it's also called the copreterite. If the other action is also imperfect, then the interpretation is that both actions were occurring simultaneously (or habitually) over the same period of time. If the other action is in the preterite, then the preterit action is interpretted to have interrupted the imperfect one:
- Él pintaba y dibujaba cuando era niño. (simultaneous habitual)
- Él pintaba mientras dibujaba aquel día. (simultaneous non-habitual)
- Él pintaba cuando decidió dibujar. (interruption)
Effectively, using imperfect never advances the narrative timeline, such that other actions occur at during the base action. Notice:
- Él pintaba ayer. (timeline is still in yesterday, absent other indications, a preterite action will be interpreted as ocurring yesterday)
On the other hand, a preterite verb does advance the narrative timeline, such that other actions occur after the base preterite action. This is why (non-habitual) sequences of actions are always in preterite, even with verbs that normally might sound odd in preterite.
- Él pintó ayer. (timeline is now past yesterday. Absent other indications, a preterite action will be interpreted as occurring sometime after yesterday, e.g. today)
- Él pintó un cuadro, dibujó algo para un cliente, y se fue a dormir. Durmió tres horas antes de despertarse de nuevo. (timeline advances after each verb: he paints, then he draws, then he goes to bed, then he sleeps. Timeline is now set to after he wakes up).
So in your example with the Maya, if I wanted to discuss things related to the fact that they were peaceful, I'd need to use the imperfect, because my statements are all based around that description's timeframe. On the other hand, if we're concerned more with the fact that the civiliation was peaceful (and now is not, or no longer exists) and we will not discuss events during the peaceful time, preterite is the better choice. In isolation, preterite sounds a bit better to my ears, but imperfect is absolutely acceptable.
