Battery life problems

I've got an old Siemens M35 that I keep for emergency use in the car.
The previous NIMH batteries lasted about 3 years. I was forced to buy
a Li-Ion battery last time as no NIMH were available and it only
lasted a few months. The manual says it is OK to use either type. Is
it something that I'm doing or was it a faulty battery ? What does the
difference in mAh mean ? Will the supplied charger cope with both
types ? Does battery quality vary between manufacturers ?

First 2 batteries -

Siemens V30145 - K 1310 - X 133 3.6V 500mAh NIMH
Siemens V30145 - K 1310 - X 132 3.6V 500mAh NIMH

Most recent -

For C35 Li-ion 3.6V 1350mAh

TIA

Daytona