Which 35 summicron is it ? I started with my M8 and just a 35 cron Asph, and I am still taking over 3/4 of my pictures with it and I have not ever regreated buying it.
Do you prefer the 50mm focal length or the 35mm focal length on 24x36 ?
I tend to prefer 50mm on 24x36 and compact lenses therefore my preference for the 35 cron. The Zeiss Biogon 35 f2 ZM has also quite a good reputation.
In general I would recommend checking out
http://www.reidreviews.com, Reid did a lot of lens reviews on the M8 which should help you in your choice and the subscription is worth it if you are planning to spend a lot of money on lenses for your M8.
For the price of your 35 cron you can get 3 new CV lenses, whether you prefer one better lens or 3 good ones depends on what you intend to do with it.