When choosing look at the slight variations in the styles. Perhaps more sparring or more Ki exercises.
For me it was the Sensei that was the most important thing. Luckly the dojo had everything else. But if you have a good Sensei then everything else will fall into place. You will enjoy it more and therefore go to more lessons. If there is a good Sensei then the other Aikidoka there are more likely to be very nice. As the pupil is a reflection of the Sensei.
If you are into reading books, try in my opionion Aikido by Koichi Tohei. In my opionion the best of the oringal "disiples" of Aikido. Gozo Shioda's books are good, but a little complex. It feels a little squahed in. All of these can be got from Amazon.co.uk or where ever you are from. Also Kissmaru Ueshiba book is very good which is also called Aikido and the current Doshu is releasing his first book soon called "best Aikido" which looks very promising.
Oh the "Aikido and the dynamic sphere" is one of the best books, you really wont find many better.
Hope this helps