100 AMP sub panel from 200 AMP main
Run conduit the whole way and use individual XHHW-2 wires

First off, if you're going to run conduit, simply run the entire run in conduit -- it's not all that much more expensive than a cable wiring method parts-wise, and running a fat (say 2") Schedule 80 PVC conduit for the whole run means you can put fatter wires in in the future if called for, as well as making the pull relatively easy for whoever (you or your electrician) does it.

As to what goes in the conduit, once you have a conduit for the whole length of the run, you can run individual wires (presumably XHHW-2 insulated AA-8000 aluminum alloy wires at the sizes you'd be dealing with for 100A) for the hots and neutral, and a relatively thin (8AWG) bare copper ground.  This is no more expensive than either a SER or a URD cable in the grand scheme of things, takes up less space in the conduit, and is far easier to pull through the conduit than a fat, stiff cable will be.