Travelling for 3 nights from Barcelona next May with 2 young children.

Anyone familiar with the difference between those two resort towns?

Very few differences...you almost can't say where one finishes and begins the other due to the construction. I like more cambrils because the actual village is smaller, and still looks a bit as it once was..but the beach resorts are both the same, overbuilt. Fortunately for you, May is still not high season so it won't be crowded.