Much depends on what you are using it for.
If you are gaming the faster the better. For storage, the larger HDDs are better. But for general use any will be fine.
However, battery life may be a control so check this. Again faster machines often use more power.
External HDDs will slow you down as transfer rates will be lower than on an internal drive. A HDD that uses laptop battery will also drain it quicker.
Perhaps you should do a full feature and use comparison for each and then add you use in. It is likely to be the best way to decide.