I'd definately go with wider tyres. I currently run 1.9" slicks on my commuter (an ex-mtb) but 1.5" tyres should be fine for most purposes. I'd go for the widest slick tyre that will fit, up to around 2". The wider tyre will run at a lower pressure and do more to help soak up bumps. If the roads are really bad you might consider a stiff suspension fork.