It's really about the meaning of 'decide'; people say they are deciding something but they're not really.. They are discussing it or thinking about it and TRYing to decide. 'decide' means make your final choice, so 'deciding' doesn't really make sense, strictly speaking. But people say it and know it means 'trying to decide'. So, it is wrong to say 'deciding', but people do - they think it means 'considering', which it doesn't! :)

According to Merriam-Webster’s Dictionary, “decide” is an action verb and can be used in the continuous form if it has the meaning of “to make a choice about something; to choose something after thinking about it.” For example, “We are deciding if we should take a vacation,” would be an acceptable usage. Note that a synonym for this sense of “decide” would be “consider.” There are other senses of “decide” that are not actions and cannot be used in the continuous form.

Yes, it's grammatically correct. You can also say, 'I'm thinking about buying either a car or a house.'

Mohammed's version or " I'm trying to decide ..." Lots of people wish they could use the same sentence in the real estate market in BC : )