Personally, I prefer a heavyweight Karate dogi in Aikido. The main reason is probably because Karate is my first love and I'm just used to wearing a Karate gi. My Sensei (Roy Suenaka, Soke) was a student of O'Sensei Ueshiba's and also wears a heavyweight karate gi to teach Aikido.
My suggestion is to wear either a judo/jujitsu/aikido gi or a HEAVYWEIGHT karate gi. However, heavyweight karate gi can be rather expensive. For a beginner or someone on a budget, I recommend a light or medium weight judo/jujitsu gi. Stay away from triple-weave--I bought one and it is so heavy it feels like I'm wearing an old-fashioned diving suit.