Which type of slide you choose to use are largely a matter of taste. Different materials produce different tones. If you lay acoustic (Dobro, lap, regular acoustic) might mean you choose a copper or metal slide, depending on style. A glass slide is perhaps the most traditional type of slide, whilst the chromed steel slides sold today are perhaps the most common ones today. I myself prefer the chromed slide, or the plastic ones. I recommend you take many different materials for a test drive, and also try them using acoustic and electric (clean and distorted) guitars.