Cockatiels and parakeets both make wonderful first time birds, but I would go with a cockatiel. Yeah I have cockatiels so I have to be a little biased. LOL J/K

The reasons I personally would take a cockatiel over a parakeet are as follows:

From what I have seen, cockatiels become far more attached to humans then parakeets.

Tiels are more responsive and seem to understand and comprehend us when we talk and whistle to them. My friend's parakeets seem to ignore her when she talks to them, but they may be different.

Cockatiels lifespan surpasses that of parakeets by 10 years, but they both have lengthy lifespans.

This is sad but, parakeets are tiny and far more prone to being stepped on accidently than a larger cockatiel. This happened to my friend's keet and the poor thing died instantly. It broke out hearts.

They are both acrobatic and playful and happy little birdies. Parakeets come in beautiful colors and cute little beaks. Cockatiels have those gorgeous crests and orange patched cheaks!

Cockatiels are very intelligent and have a greater chance to learn tricks, how to whistle tunes, talk, and dance than parakeets.

I love them both, but cockatiels rule!

Some good websites you can visit for information include:

www.cockatiel.org
www.cockatiel.com
www.birdsnways.com (this site has great articles and several forums including a cockatiel one. If you go to forums and click on Tiel Talk, you will find a cockatiel forum as active as this one. It has several moderators and breeders, and other people that are very helpful for advice. Its a great board. Just join in a post your questions, and hopefully you will get responses from people that know more about birdies then me. Good luck, and Logan's right about getting an avian vet before hand. Its important to have a vet before needed and its best to take the bird for a well birdie exam right after you get him/her.