The primary difference between Campden tablets and powdered K-Metabisulphit is that Campden tablets are Sodium Metabisulphit as opposed to potassium metabisulphite (powdered form available at your LHBS)
Apart from that 1 campden tablet per gallon will add sulphite at a rate of 75 ppm. You're also adding sodium to your mead/wine when you use Campden tablets, which can become detectable by taste especially in white wine/mead. Depending on the amount of sulphites already present from your fermentation I prefer to use the powdered K Metabisulphite because it's more exact (since I'm weighing it out), it doesn't add sodium that may become detectable in the flavor, and the K Metabisulphite will help during cold stabilization (Tartaric acid will precipitate out and tartaric acid cystals will form) rather than during long term aging in the bottle or carboy.
When using either form of Sulphite you must use pH to determine the amount of sulphites you'll need to add to reach your target level. When I do add sulphite it's generally 30-35 ppm.
Hope that helps,
Oskaar