I've wanted to own a Leicaflex SL for years but when I got the Canon New F-1, i never looked back. A friend of mine -camera collector- owns several Leicaflexes. They feel really great in the hand and are extremely well made. But i dare to say that the viewfinder on the Canon F-1N ("New F-1") is brighter. It is the best viewfinder i've found on a SLR. Plus the shutter and mirror shock is really, really smooth, as smooth as it gets, I dare to say. You can also have extremely precise spot metering on the F1, by the way.
It's not necesarely the brightness that made the viefinder stand out, it's the magnification (0.9 - same as M3), chrispness and the pop effect when the image is sharp. Easy to shoot with a f/6,8 telyt. Bare in mind that the viewfinders are half a century old now and that many didn't age well; desilvering and brown spots are common.