Not sure what you want to do with your work bench.. if it'll be a work bench for wood working, an assembly table more along the lines of a catch all bench for sharpening things, working on parts or maybe just a flat place to do gluing jobs etc.
For wood working you need ridgid and flat.
Mortise and tenon may prove frustrating just starting out, and while screws can work I think a good way to look at most any supporting joint esp holding the top up - avoid just screwing a 2x4 across the sides of posts. Other than timberloks, most screws are quite brittle and don't handle shear forces well. They're meant to hold 2 pieces together, not support weight directly. Instead if you can, think along the lines of using 2 2x4's (or whatever material) glued and screwed together for an upright, one cut shorter than the other. Creating a notch to rest a load bearing cross piece on, then screw together. Then screws aren't bearing the weight.
The first image is not a very strong connection, it relies solely on the screws. The second image is a much stronger connection, fully supported with the screws just holding pieces together. (just as something to consider with pieces which will bear weight, regardless of your actual design)