90 watt or 120 watt! power adapter??

No harm in getting the higher rated one.

Indeed. As long as you get the correct voltage, you should be fine, regardless of wattage.

On the bottom of the laptop it should say how many volts and amps it needs ie it may say 18.5V 4.9A if you multiply these together you get 90.65 which is the watts. You should get a power supply that has the correct voltage and at least the correct Amps/Watts.

The problem, as Simon12 eluded to, is that some Bricks only provide the stated output at the stated Current. When the current is less than the rated value, the output voltage is higher. In the case of Laptops the regulation is probably in the Laptop and if that is the case then no problem using the larger (120 Watt) as the current (output power) will be the same for either the 90, or 120 Watt brick. The 120 Watt should last longer and be less prone to failure when used at a lower power output and my choice - However the 90 watt should also work fine. You should not use a 90 Watt brick if you need a 120 Watt brick.

You might check out the Battery pack, this could be the reason for two different power adaptors. Ex. My laptop comes with a standard battery; However there is a extended battery pack. The extended battery can be charged using a higher current than the standard battery. I use the same brick for standard battery, or the extended battery. 19 V @ 3.95 Amps ( approx 80 Watts)

While I talked about charging currents, The max output requirement is based on Charging current required by a depleted battery pak + Operating current required by the laptop under full load.