Difference between SN754410 and ULN2004
H drivers are usually used for circuits requiring a need to entirely reverse currents to some element. This is usually seen in motor drives. Having an H-drive (or H-bridge) allows one to source current from either side of the motor and sink current from the opposite side. This configuration usually allows one to shut all circuit elements off as well which allows the motor to freewheel so that it's not having to always brake, or accelerate.

The ULN2004 is an array of seven NPN Darlington bipolar transistor pairs, so this is meant to be able to sink current. If it can only sink current, then the other side of circuit element in question (light, motor, or some other high current device) may not be reversed. It can be shut off and turned on only. This is usually not useful in many motor cases where you want reversibility. On the other hand, it's very useful and works fine for devices where polarity doesn't matter: things like LED's which can't be driven backwards anyways or a motor in a device like an electric razor.