You're right; it can be confusing. The differences between the preterite and imperfect tenses of ser, especially, can be subtle. In sentences where either verb form could be used, the difference might not be translatable to English without adding context.
Just one point before going on to the main part of the answer. While amistoso usually is translated as "friendly," it usually is used when describing a thing (such as a game, attitude or relationship). In this case, you'd probably want to use amable or recast the sentence to use a tense of ser amigo de.
The quick answer to the main part of your question is that era would probably be used in the sentence you gave, because you're probably talking about something that happened (or, more precisely, something that was) over an indefinite period of time. If you were to modify the sentence (or provide a context) to indicate that the neighbor was friendly only during a specific time period (for example, fue amable conmigo durante esa semana), you would use the preterite form.
As a general rule, you're normally safer using the imperfect of ser (yo era, tu eras, él o ella era, etc.) unless you're clearly talking about a specific time period (as when following the verb with durante).
Normally, ser would be used when providing the background to the events of what you're talking about. If however, you are talking something that represents an action, the preterite would be appropriate. For example, if you were talking about your friendly neighbor, and there was a specific time when he became a grouch, you might say fue refunfuñón to indicate that the grouchiness was an event, not a mere background to your tale.
You could also use the preterite to indicate something was complete. For example, estuvo lista, she was ready. Similarly, the preterite could be used to show that something had a definite beginning and/or end. Note, for example, the difference between estuvo enfermo (she got sick) and estaba enferma (she was sick).
Another use for the preterite of ser, especially when used preceding an infinitive, is to indicate that something actually happened. For example, fue difícil nadar (it was difficult to swim) suggests that someone swam with difficulty, but era difícil nadar indicates that conditions were difficult for swimming without indicating whether anyone actually swam.
