I don't think I've ever seen, let alone bought, a RS guide so I can't make a comparison.
But I think the Bradt guides are the best available. However they don't bother to cover a lot of Europe, being more focused on counties in Britain and specific areas of the rest of the world. So for elsewhere I think the Rough Guides are a good stand-by, ideally complemented with a DK guide (which have a different style, being more pictorial than wordy). I avoid Lonely Planet books since they are poorly written, but they essentially cover exactly the same as the equivalent Rough Guide. For a long weekend-break style trip to an individual city, the Insight Explore Guides are very helpful.
I think, overall, you should go to your local library and see what type and style of guidebook you prefer. Some people like to be spoon-fed the "must see" places, others prefer the authors to lay out what's available and leave one to make an intelligent choice (or guess?) based on your own interests rather than the writer's. No single approach will suit all.