I've not participated in either program.

The difference to me stands out as committee vs opinionated.

With YC a small handful of people give you advice and nudge you in the correct direction. Seedcamp has dozens. One participant boasted to me that they had met 100 mentors in a week.

I much prefer opinionated software over committee software. I imaging the same is true for startups. Surely 100 mentors will give you 100 conflicting pieces of advice. No matter how smart those mentors are (and everyone I know who is involved in SC is smart) they can't help you out if you are confused, disoriented and overwhelmed.