I'm a bar bag convert. They don't make steering any heavier, ime (well, they might if you fill it with bricks!) and are just the thing for the odd bits and pieces you need on a ride; cameras, biscuits, arm warmers, biscuits, gloves, biscuits.....
Good support and fixing is essential ime. The rixen paul method is good and solid, there are others. Avoid cheap looking fittings, they'll only bite your bum later. Also useful (if you have more than one bike) is being able to buy the fittings separately, allowing you to easily use the bag on other bikes.
Another thing to check (if you have drops) is the width of the bag. On narrower drops (38 or 40cm) some bags could leave you struggling for hand space, particular if the bar bag has 'useful' pockets on the side! (Pockets, particularly external ones, can actually be more of a pain than an aid ime, a simple box style bar bag is perhaps the thing to look for)
There are any number of makes that'll do the job, I have no personal preference. I just went into my LBS and bought the one that looked the most robust and gave me what I wanted storage-wise. It had a plastic map cover thing on the top but I've never used it - again, it's more of a pain than a use!