More expensive racks will be made of steel or even titanium and will be relatively lightweight (though still heavier than an alloy rack)
Cheaper racks come in 3 flavors:
-alloy racks (a great bang for your buck)
-cheaper steel racks (VERY heavy)
-even cheaper cruiser racks made of sheet metal (terrible in almost every way and usually only fit the bike they came on)
maybe some more too.
As mentioned above, avoid less legs if you're using panniers. Less legs can leave the bag unsecure which could cause it to shift during riding. One has to ask if saving that extra few grams is worth the loss in peace of mind. More material (in general and all other things equal) can usually carry more weight or deal with torsional forces better.
I opted for an axiom journey rack. It's a beefier alloy rack. If this breaks down the road I'll more than likely go with a steel tubus rack. Just wasn't in the budget at the time.