I am a accredited viessmann installer and can only urge you to go for the 200. Had another customer without hot water and heating this weekend after the spark gen on a 100 failed after 18 months.
Common fault.
Cut away all the old silicon. That means ALL of it. Scrape it ALL away with a stanley knife blade if need be.
But get rid of ALL of it.
Reapply a nice bead of 786 as rec ommended above.
And Bob is your mother's sister's husband.