The mah ratings imply a linear discharge. The reality is the stored energy loss is not linear, a heavy load drawing several hundred ma or more will dissipate more of the stored energy than an equivalent few hundred ma load over a longer time period. If you read the fine print on some battery spec sheets you'll see something like "2500mah" rating (at 200ma max. load). The higher mah rating of NiMh will not be realized unless the load is limited to a few hundred ma.

NiCd has the lowest internal resistance and can, therefore, deliver higher current then a NiMh of the same or higher mah rating. NiCd will work better for intermittent heavy load currents but you pay for it in weight and have to deal with the nuisance or cycling them every month or two.

The main advantage of NiMh over NiCd, and reason for their development, is the environmental impact. Cadmium is a toxic "heavy" metal that, in many countries, is banned or must be disposed of properly or face heavy fines. NiMh is environmentally friendly but has the highest internal discharge rate in addition to higher internal resistance. Leave a fully charge NiMh sit for a few days and you'll bleed off half the charge.