Definitely 'Make offer'. If an end-user looks up your name on Sedo, they'll get the name for $5k (your fixed price), when they was intending to offer $20k.
Anyways .. If you want to set a fixed price higher than $10k, you need to contact Sedo first for them to 'approve' it .. I usually keep 'make offer' on all my names, but it's good to set a minimum offer so you get rid of those who offer $100 for your LLL.com's.