Is it better to daven the shemona esrei quickly and have less (and possibly no) external thoughts or is it better to daven slowly and more thoughtfully but have (or run the risk of having) more external thoughts?
Mishnah Brurah Orach Chaim ch. 98 delves into a somewhat detailed discussion of the type of kavanah required. Basically, he emphasizes 2 principles:
- To concentrate on the meaning of the words
- To imagine that he is standing before a human king, and therefore he would concentrate hard on what he says so that he avoids stumbling
The commentaries elaborate further on these areas.
IMO, davening pace does not necessarily correlate to kavanah, in either of the two forms that you mentioned, nor regarding the requirements mentioned in the Shulchan Aruch. I have seen many people who understand the words well that they can daven quickly while still concentrating on the word meanings. Contrarily, there are those who daven quickly and are really concentrating on "going through the motions" so that they can catch the train to work. (Many of these people are shlichei tzibbur, which exacerbates the problem!)
The answer to your question, essentially, depends on the individual, and if you read further in Shulchan Aruch, he implies that one should avoid ANY external thoughts. So, it seems that's the priority, but it has nothing to do with speed.
A friend of mine once asked R' Ezra Neuberger for advice on how to daven with more kavanah. His answer was "daven fast." Apparently, he held keeping focused is the more important kavanah.
The Rambam (Hilchos Tefilah 4:16) defines kavanah as removal of external distractions:
כיצד היא הכוונה--שיפנה ליבו מכל המחשבות, ויראה עצמו כאילו הוא עומד לפני השכינה
What is kavanah? That one turns his heart away from all [other] thoughts ...
So it seems that this is the primary kavanah.