Perhaps the most important thing I've learnt here is that it is not about the glass. The Leica body limited controls and ergonomics, including not even using the storied rangefinder is what it is all about. And so here I learnt about non-Leica lenses. Quelle horreur.
I have one Voigtlander lens a 25, tiny and with a neat screw on hood, but it flares; and five Zeiss lenses, along with several Leica lenses.
The 50 C Sonnar bowled me over, and still does. So much cheaper and more interesting than a Summilux. (I had a version 2 for a time, stolen.) Only with that lens was I introduced to something that I didn't know of much and had little interest in, the character of lenses. Now I do appreciate it, this occasionally makes a difference to me, but the ergonomics of f2.8 and slower lenses are much more important to me for ease of carry, and then being inclined to carry the camera at all.
My version 4 Canadian 50 Summicron I've had for forty years is perfect in terms of size and ergonomics, the focus tab (bear claw) and the reversible hood with cap. It flared once in late afternoon sun high above the sea. The black and white negatives from it on Ilford FP4 and the Kodachromes were all wonderful. I did take that for granted. But in your position I'd be happy with the Planar. No doubt you've looked at the 50 Planar thread here. (see link below) Wonderful shots. My ZM 21 4.5 is likely better than the Leica equivalent, except for purple fringing, and the 35 2.8 is a gem, and the ZM 25 2.8 is the sharpest lens I have. I love the Zeiss colours of these lenses and the four I've mentioned have their own characters as well. It's possible you'd be disappointed with Leica's colours out of a 50 Summicron. My Summicron 35 is not better than my Zeiss 35 and with digital we don't need f2 and f1.4 lenses. I do have the DR Summicron 50 which has many great qualities, lightness not being one of them. Somehow red roses are captured best with that, even after indifferent scanning.
If you are more into photography than into lenses you can be happy you're not missing much at all sticking with the 50 Planar. I enjoy riling Leica insiders who notice me in the street with the wrong gear, a silver lens on a black body, or once, the M5, hanging vertically with the 25 Zeiss finder and the little Voigtlander 25 lens cap. But the Zeiss lenses are handsome machinery with their bayonet hoods, the C Sonnar perfect on the M5 or M9, and the ZM 25 too, not a small lens. Aperture for aperture (as in pound for pound) Leica is smaller than Zeiss.
PS The Crazy about the 50 Planar thread:
Drink & Be Merry...
www.rangefinderforum.com