Hello Steve
I've got the same dilemma, also doing Mk with Paris on the 4th and tempted to go for a PB on a flat course. My last long run I had to cut from 20 to 16 miles and I've not properly recovered yet. My PB is 2.04 in Amsterdam (flat) and 2.06 here. I've run 2.07 for 13 during a recent 20 on undulating roads, I'm really tempted but as I'm tired at the moment I don't think its a good idea.
I think you need to decide which PB you want the most and if going for the half might ruin Rotterdam, PB or not.