Using Battery (li-ion) with higher capacity
Yes, it is safe.

What is important in supplying power in batteries is the voltage.  As long as you can supply the same voltage, then different capacity batteries will work, the only difference being different run times.  

A similar basic logic applies to power supplies as well.  If your laptop has a 19 volt 1.5 amp power supply that you plug into the wall, you could replace it with a 19 volt 5 amp power supply. The laptop would only pull or grab the 1.5 amps that it needs, the power supply does not push the current into your computer.  The voltage however, should stay the same.  You should not go with a 19 volt 0.5 amp power supply as the laptop would try to pull more current than the power supply is built to supply and it would overheat.  I am getting a little of topic.

Go for the higher capacity battery if you want to spend the money.