All radar detectors these days, detect the same thing. X, K, Ka, Superwide, and Laser detection can be found on radar detectors costing as little as $30.
The difference between a $30 detector, and a $300 detector, is the quality of the internal components. A cheap detector is gonna go off anytime it senses some form of microwave. THat could be a cop, or it could be an automated door at a WalMart, or even some moron nuking a corndog at 7eleven. A good detector will be able to discern the difference amongst all the aforementioned, and it will give you more range (time to slow down) as well.
Basically, if you spend the cheese and don't have unrealistic expectations, it will work well, and ultimately pay for itself. If you go cheap, it will become more of a distraction than anything else.