Formula using arrow icons to show trend

I'm trying to create a formula where I can show the trend of two values from one month to the next that shows wheather the results went up or down in a positive or negetive value. For example:

If I have a transfer % of 18% in October and November MTD is at 17% - the arrow would then reflect downward showing in green, as the lower percentage is better. but I need the arrow to show upward in Red if say the value for November MTD was 19% instead.

I tried conditional formatting and attempted an "If" formula. I must not have all the information to create the formula as it errors out on me. I can create it if only looking to reflect one arrow...

=if(c3<b3,"↓")

Any help is greatly appreciated.

FYI - I have excel 2007.