Hi Missprim,
BHP is simply a measure of the power of the engine. A car with 123 bhp is more powerful than one of 96bhp. That might be important if you want to drive hard or are cruising long distances on the motorway but not if you just want a car to potter around town and cruise along at 56mph on the A roads. However, the more powerful a car is (the larger the engine size which is generally reflected in the bhp) the more expensive it is to own - road tax will be higher as will the insurance.
The CO2 levels are a measure of the pollution emissions coming from your exhaust. Vehicle engines must be in sufficiently good condition that they burn fuel to acceptable environmental standards. The lower the CO2 figure, the cleaner burning and better serviced is your engine. The higher the figure the more the engine is worn or poorly serviced. It's only something to consider at MoT time as the car will fail if the emissions are too high.
The CO2 figures you give are quite good. At the end of the day every fossil-fuelled car pumps out very nasty poisons from the tail pipe. If you run a hose pipe from the exhaust of any modern car to the interior you'll die just as fast as you would in a car that's fifteeen years old! It's rather meaningless when people say one car is 'cleaner' or 'greener' than another. Anyone seriously worried about the environment rather than just saying they are would walk or go around on a pushbike! The emissions levels only really matter at MoT time.
In short, the car with lower emissions and less power will be cheaper to own (fuel, tax and insurance) than a more powerful car. I hope you find the car you are happy with. Good luck.