Hi,
I would go for the 501 or 503. Even if you don't need TTL, the viewfinder is better. On some 500CM, if you put long lens, you don't have the full image on the screen.
It's bulky, less so than SL66 and the shutter sound is quite loud. The SL66 gives you incredible close up plus tilt possibilities.

I have a 503 with 60mm and 100mm. I use it only on assignement. I really miss the SL66 (sold long time ago), and I'm more incline to use a Rolleiflex on a daily basis. Quite and discret.

It depends on what you exactly need and which lens you want.