If you were to have only one, it should be radar, but if you have radar, adding AIS (even receive only AIS) helps immensely with collision avoidance. Here is how: Radar, in its most basic application, just gives a static picture of other vessels around you, but it doesn't tell you anything about risk of collision. Most radars allow you to add an echo trail, and that helps, but not much. Good radars have ARPA or MARPA, which basically tracks other vessels and calculates a TCPA and CPA (time to closest point of approach, and closest point of approach), and you can set collision avoidance alarms to warn you if you are within say 30 minutes of being within say 1nm of another vessel. It actually takes a fair amount of work to monitor, especially if you are nearshore with lots of traffic and have to set the targets individually. Conversely, AIS gives you an instant readout of each AIS equipped vessel, and the exact same ability to set alarms. Moreover, you can (or at least should) be able to overlay AIS on your radar, so you can see every AIS target's radar blip. Blips without corresponding AIS icons (ie, the other boats out there without an AIS transmitter) are easy to spot and keep an eye on with ARPA / MARPA.
The farther offshore you go, the more useful the radar AIS combo becomes (inshore has just too much traffic). So, I guess I should say, if you intend to use the boat within about 5 miles of shore, you probably won't get much out of AIS that radar won't give you. But if you are routinely crossing the catalina channel, or beyond, I believe you will find it worthwhile.
