A) Wiktionary: http://de.wiktionary.org/wiki/er-
[1] Vorsilbe, die ein Nachgeben oder unter Belastung Verändern ausdrückt
B) Wiktionary: http://de.wiktionary.org/wiki/auf-
[2] Ein Wortteil, der ein Ereignis beschreibt, bei dem etwas ans Licht kommt.
[3] Ein Wortteil, der einen Öffnungsprozess markiert.
Also if you are talking about somebody in the 3rd person like in your example. Aufwachen would not sound good. Also its in the third person past tense.
Er erwachte.
but
Er wachte auf.
Erwachen would be simpler in this case. Your example is related even though
"Als Gregor Samsa eines Morgens aus unruhigen Träumen erwachte, ..."
is close to
"Als Gregor Samsa eines Morgens aus unruhigen Träumen aufwachte, ..."
Yet erwachte is easier to roll of ones virtual tongue than aufwachte.
Both have three syllables but.
erwach is one breath out and the the second breath out
auf is the first breath out, stop, then wachte is the second breath out.
The stopping in the middle even if you just think about it or have to think about it while reading is just an obstacle.
@embert is correct however in that I'd never say ich bin heute früh um 7 erwacht. Why?
Syllables and breaths again. While when reading a smooth flow is good. When you speak the pause gives the listener time to process and think and the speaker a chance to pronounce the word better for better communication. Try pronouncing erwacht bin vs aufgewacht bin. One you can literally shout while playing with the intonation (theatrically) the other not so much although it can be done but whith more effort and it feels less natural. That's why its better used in writing.
I hope that was clear thx.