Rangefinders and SLRs do their own particular things very well.
For me it's not a question of "either or" but I'm happy to have both to use as needed...
You'll also find excellent lenses in both worlds, though it's usually easier to find good SLR lenses at bargain prices.
Unsuprisingly, rangefinders often have an edge when focusing wide angles in gloomy light, while SLRs with good viewfinders let you see a longer lens "snap" into focus better.

In the SLR world, if you want the best and snappiest manual focusing there is (like an H screen, but *far* better in all respects), hava a look through a Leicaflex SL viewfinder. The camera even has a spot meter and better dampened shutter and mirror than the F3...