Bristol probably has the better reputation for Politics, but overall this is more for political theory than international development.
Bath has a very good department, plus it is a much nicer city - fantastic place.
I would go through the course outlines and see which course appeals to you most - both have a good reputation so pick the one that is more focused towards what you want out of it. If you're interested in prestige of the institution, probably choose Bristol (but take lots of day trips to Bath whilst you are there!)