You didn't post what type of a Mac you have. The answer may be different if you have a Powerbook vs. a tower.
The obvious answer is: People doing anything that relies on specific software (such as audio, professional graphics, video, scientific) should not adopt any new operating system until their key software is fully tested and compatible.
So if you already have a Mac on Panther, DON'T install Tiger yet.
This presents more of a problem for those who wish to buy a new Mac, when all new Macs come preinstalled with Tiger. There is a limited opportunity to buy remaining stock of older machines when the new OS gets bundled (that's what I did for some clients on ProTools). Beyond that, you're faced with reinstalling the older OS or waiting until the applications are upgraded.
> Should I buy a different input device and just uses garageband?
Depends how much time and effort you have into ProTools. If you're a newbie, then you can make the switch and it'll just cost some money ($300 - $1000). If you have been using ProTools a lot, you may be aggravated by the differences. You are not going to be able to open ProTools projects in another program, you'd have to export and import the tracks.
> Also, since I have 6.2, is there any way to get an update for tiger when it does come out?
You'll have to ask Digidesign for their policies. Past upgrades of ProTools have been in the order of US$75