Non Integer Quantities in Craft Commerce
Given how much of Commerce internally will be relying on whole number quantities, I think option 2 is the far better plan. 

You can't add custom fields to line items currently, and while you might be able to hack something using the options/notes systems, or custom order fields on the order as a whole, I'd think this would get complex to maintain quickly. Best to use your minimum selling measure as the unit and dress it up front end.... that should work well and will be substantially easier to implement and (importantly) maintain. 

With stores, even more than websites, long term ease of understanding and maintenance should be a high priority goal.