Well, I don't have many details for you, but this is what I gather so far...
Laser and IR will give you the smallest beam width and highest pulse rate, which normally translates into better resolution if you're using a sweep pattern. The receiver would have to be specially designed for outdoor uses or else there's a great chance the signal will be masked by sunlight, even if you modulate it. A nicely-focused laser like the type used in police lidar should scatter well enough return from some targets. Without a rangefinding function, the sensor will be misled by target color.
Right now I'm experimenting with a sonar device from EDPCompany.com called "SonaSwitch Mini A/S." It's an embedded sonar and it can reportedly detect objects as far as 40 ft, once configured for that. The "ping" rate however, is much slower that what you could get from a laser, so that is already extending my sweep time. The echos do not scatter as well as light, so some targets will remain hidden.
Well, I guess this isn't too imaginative, but my advice is to use both in a redundant fashion. There are a lot of UGV competitions on the web, and generally the ones that perform the best do so only because they are able to combine sensing methods.