A shield is equally effective at all levels, even low levels. I think it's like a 40% deflection bonus for round shields.
If you are tanking, I highly recommend using a one handed weapon, and a shield.
If you aren't tanking, use a two handed weapon for faster hits and more damage.
As of yet, I can tell no difference in effectiveness between slashing or crushing weapons. I would suggest keeping them both up, by carrying 2 weapons. A two handed sword, and a one handed hammer and round shield, for example. Though if you wanted you could go with a two handed hammer, and a one handed sword and shield. Either way works.