Chemistry, power density (capacity per ounce or per cubic inch of battery), power dissipation when not in use, lifetime based on # of charge discharge cycles.

The NiMh and NiCd nomenclature refer to the metals used in constructing the batteries. (you can look up the exact chemicals and metals for making the things...)

NiMh has a higher power density than NiCd. MUCH higner by the cubic inch measurement, not quite as high a difference by the weight measurement (NiMh cells weigh a bit more than NiCd per cubic inch) but there's still a considerable benefit in weight savings to get the same capacity.

NiMh tend to hold thier charge better while sitting on the shelf.

NiMh have a somewhat lower number of useful charge-discharge cycles than NiCd. the NiCd will seem to have suddenly gone bad (over 50% capacity loss in one cycle is typical. Trash/recycle them when this happens!). the NiMh (my resutls...) seem to drop in capacity "slowly" (dropping 10% capacity is the warning... just 5 or 6 more useful charges at decreasing capacity) when approaching end of useful life.