The age old question about little kid sleep: cry it out, or cave and soothe?

Recently transitioned 23 m/o to toddler bed (had to, she could no longer be contained). Great sleeper is suddenly a hysterical nightmare unless parent is lying in bed next to her toddler bed/crib. The age old question when kid is sucking at sleeping: better to let them CIO/deal with it, or cave and do what it takes to soothe them and hope it's temporary?