what is the difference between these two configurations?
The two circuits are both based on emitter followers. As you may recall, the emitter follower has a gain of about 1 and an offset of one base-emitter voltage, Vbe. The followers are arranged as a complementary pair.

In both cases the purpose of the diodes D1 and D2 is to insert a bias that parallels the Vbe of each transistor. (Vbe is about the same as a diode drop, 0.7V.) Since there’s two Vbe in opposite polarity, the net difference between the two bases is 2*|Vbe|, or about 1.4V.

As each circuit operates, the top and bottom of the diode pair track the input, with an offset of 1.4V with respect to each other. In circuit 2, the input is simply driving Qp as an emitter follower directly, Qn’s base will be 1.4V above it.

The second circuit has the advantage of being DC coupled, though with an offset of one -Vbe input to output. The first circuit on the other hand is AC coupled, but is self-biasing for single-ended operation.

Try simulating this in LTSpice to see what is going on. LTSpice is free to download and has a good library of parts to work with.