To make sure that the HTML newsletter looks as close as the designs I would definitely use inline styling.
Unfortunately you have to be very specific with your styling, otherwise some software (Gmail for instance) will override your style declarations. It is a real pain sometimes, but it's worth doing it that way IMO.
Same thing goes for HTML email signatures.
So definitely go for the inline styling :)
Oh, and one more thing. I know that it's 2012 and loads of designers/front-enders advice to use html tags (even html5 elements) in HTML emails and signatures, but I'd try to stick to the old-school way and use tables to make sure there is no unnecessary artefacts with the code.