Comparing Radar to AIS is not really very valid. You did not describe the type and location of sailing you plan on. If you are sailing in areas that have limited fog, then Radar is much less of a must. Boats have been cruising for a long time without Radar. It is only in the last 10-15 years or so where small Radar prices have come down so much that many more boats have Radar. On an overnight passage it is nice to be able to turn on the Radar and see whats out there and what way they are going. Get caught in fog a few times and you'll see the value very quickly.
AIS prices have come down so much that it is a nice safety feature, independent of Radar. Most can be left on continuously with a much lower current draw than Radar.