Best way to meter light for LCD led dimming?

I'm in development of a motorcycle on-board computer based on Arduino and MPGuino project. I will use a graphic LCD with touchpanel. I want to change the level of its back-light LED automatic based on light intensity. And here is my question: I'm not sure what's the best way for metering light level. As I know I have two possibilities:

.) Using the TSL230R (light to frequency) with an ext. interrupt. But I'm afraid it will interrupt my program to frequently which may result in lost injector pulses (=less precise). Datasheet says it's output frequency (depending on multiplier) is around 10kHz! :-/ As I need int0 and int1 for injector signal (ramp triggered), I could only use an PCINT for the TSL230R. So my interrupt would happen at every signal change (type "CHANGE"), because PCINT don't supports rising or falling, isn't it? .) Using a simple photocell with a voltage divider and an analog input pin.

I'm new to microcontrolers, so I hope somebody can give me a useful hint :)

I presume that the intent is to turn the LCD light level up during the day, and down at night.

If so, the light level should not change rapidly. I'd use a photocell and a voltage divider. Check it occasionally, and adjust the light level accordingly.