Drop Spindles are a great way to get started with spinning. Drop spindles are very portable and can easily go with you anywhere whereas Spinning Wheels are not as easily portable. Drop spindles come in all shapes and weights. Depending on your experience and what type of yarn you are wanting to spin will determine the weight of the Drop Spindle.
Drop Spindles come in the following weights:
Lightweight - .4oz to .9oz
Medium Weight - 1 oz - 1.7 oz
Heavy Weight - 1.8 oz - 2.4 oz
Think of it this way. If the drop spindle is heavy it is going to want to pull your fibres onto the floor when you are trying to draft and that is no good and very frustrating. So fro shorter finer fibres, you want to use a lightweight spindle. These would include angora, silk top cashmere, camel, etc.
You can use any size of drop spindle for longer fibre such as Merino, Alpaca and Llama. If you are wanting a finer spun yarn you will use the heavy weight spindle. Keep in mind that these fibres are pretty slippery and can be hard to spin especially on a heavier wight drop spindle. I would recommend buying a hand dyed roving. I find that they are a little bit felted and the fibres don't draft as easily which is not a bad thing.
For a courser wool like Corriedale, BFL, Coopworth, etc, I would use a heavyweight drop spindle. They are not as slippery and will not draft as easily. When I say will not draft as easily, I mean thay the fibres will not slip and slide around as much which is truly easier for drop spindling.
Drop spindles vary in size, weight and color.