Hi Graeme;
Here's my humble opinion -
M6TTL - finish is your personal choice. I like black as it makes the M6 look like a non-threatening & inexpensive point & shoot when on the street, but other than that minor point, I think all of the M6 finishes look nice.
50mm - The Summilux is the only current 50mm I haven't used.
Can't go wrong with the Summicron - it's a really great 50mm, I think one of the finest. It's compact, light & with a handle, easy to focus. The previous barrel design (I think they stopped making it in '95 or so) had a focusing handle & separate hood, and it's lighter too, but the optical formula is the same as the current one. If you can find a nice one, go for it.
The current version is a little heavier, no handle (I had one attached) & built-in sliding hood, but I find its focusing action to be really, really smooth. Ask yourself if you really need the extra stop that the Summilux provides. If not, go with the Summicron.
The other 50mm lens I have is the Noctilux, but I would NOT recommend that as your first Leica M lens! It's big, heavy, the focusing barrel is relatively stiff. Great lens when you need f/1, but for everything else its overkill.
35mm ASPH Summilux - haven't used the 35mm ASPH Summicron, but I had the 35mm ASPH Summilux & the previous 35mm Summicron.
I agree with Vajirune - the ASPH Summilux is truly a great lens. I also agree that it's quite large compared to the previous 35mm Summicron. Handling the previous 35mm Summicron with its compact size & hood was really easy. It can even hang underneath an open jacket on an M camera & it's difficult for others to see whether you're carrying a camera or not.
50 + 35? I used to use the classic spread of 35, 50 & 90 lenses. With 35mm, first I had the older Summicron, then the Summilux ASPH. However, a few years ago, I acquired the 24mm ASPH. I found that I used the 50mm & 24mm almost all the time (with the 50mm getting the lion's share of use).
After that, I sold off the 35mm ASPH & picked up a 90 ASPH. The 35mm & 50mm were just too similar, after I got used to the 24mm.
IMHO, I would choose 50mm OR 35mm, but not both. When you settle on one lens as your base, that focal length dictates what makes sense for your next lens(es).
If the base lens is a 50mm, then (for me), I like supplementing it with a 24mm, & a 90 or 135mm.
If I had chosen the 35mm instead, then a 21mm & 75 or 90mm would work well.
Of course, the other alternative is to buy every focal length available for the M
...but that would be rather heavy to lug around!