The first thing to do is to educate yourself as to what digital scanners are out there, and find reviews. You can do that by using the wiki - hit 'wiki' in the top blue toolbar, then look on the right at the resulting page. The section labeled 'scanners, receivers and professional radios' is in a yellow box. There's a link for 'digital trunktrackers' which lists links for all the current (and future - the PSR-600 isn't out yet...) digital scanners. Click on the one you want to read. There's also a section a little below that called 'reviews'. Our own safetyobc has written a great many, and there are user opinions posted on EHam.
73s Mike