You'd need to get the quotes.
With the extension, depending on the size, you might have to get a planning permission. This will take time and cost more.
With the loft conversion, if you're not actually building a new separate floor but converting to existing height it's likely that no planning permission will be required. You might also be able to put dormers in, depending on whether this falls within the permitted development.
I would get an architect to draw up plans for you in line with how you want to live in this house - well worth the money. Then speak to your local authority re planning permissions required (if any) and get quotes for both, extension and loft conversion from your local builders.
In my experience and based on what several builders told me, a loft conversion can work out cheaper, make the house look and feel better and it can be a better investment. BUT this depends on a particular house - I don't know what your loft looks like and how easy it would be to convert in the first place.
I think you'll need to go with one or the other (conversion/extension) because adding a third bedroom to the way this house is atm would compromise other rooms IMO.
Also agree with pps - way too many doors! Some doors and most likely walls too should be removed to create a better flow.