Usage of "in" vs "with"

In the American language, it is a usage standard to express an interest in something. As i scan the senteneces, number 2 seems awkward. Were i to encounter that sentence standing alone, without the comparison to number one, i would think it awkward and likely written by a non-native speaker of English. Although i have no doubt that others may arrive to dispute what i've written (to which i will be supremely indifferent), i would suggest to you that as you are speaking of persons ("you," "person," "candidate," "us," "them") with would only be used to indicate that persons were in company with one another, or were acting in concert with one another--that is to describe a relationship of physical proximity. Persons might be in agreement with one another (mental proximity) but would have an interest in common.

I just love the obscure inanity of prepositions in English and American usage.

My vote: IN
I agree with Setanta here, I (British) use 'with' when both parties are working together or against each other.

If I am interested in a candidate then I am performing the act of 'being interested' on the candidate (this doesn't sound quite right, but it's almost what I mean :wink: ).

If I am performing the act with the assistance of the candidate, or the opposition of the candidate, then I would be likely to use with.

Some examples:
I am interested in the candidate's progress
I am sitting in the pub
I am sitting, with the candidate, in the pub
I am arguing with the candidate
I went fishing with the candidate last weekend

Clear as mud...

Don't know whether you need these, but...

'I am concerned with grammar repair.'
'She is preoccupied with her own interests.'
'They were afflicted with palsy.'

You could say that you are interested "with" someone, but it would have be in the context of "I am interested in (doing something) with him/her". If you wanted to be poetic you could change the order around to put the with in front of the action. Otherwise, I think you would say "in".