Your surname, 100%
For a start, it's tradition. Babies have always had their mothers' surnames... it's just that most mothers were married and had taken their husband's name.
Secondly... you're not with him. YOU will be the primary carer. The baby will primarily be in YOUR family. You're giving birth. You're caring. You're the one who will be dropping off at school, registering with doctors, everything probably...why on earth should he have more right to be the one who shares a name when every single aspect of everyday life will not only be mainly with you, but will be easier in many respects if you share a family name?
Don't give in to shut him up. It's really important, and it's about your DC too.
I bang on about this a fair bit as I have a good friend who really, REALLY regrets giving her DC her ex's surname.
The child hates it as they have a different name to their mum and their siblings. It was particuarly difficult when the sibling came along as that sibling DOES share a surname. If she'd given DC her surname it wouldn't have been a problem either way as she would either choose to give both her surname, or if she'd remarried/taken another surname then DC could also choose to change it. As it is, the ex won't allow any surname change.
Her ex's family also use it to 'mark their territory' - especially now that she has moved on and has a new partner and child. It upsets the DC (their grandchild) too. Things like 'Proud of youngest 'Smith' today for getting their swimming badge' on FB - they never just use the child's first name. No matter that the father sees the child once a month and is shit, frankly.
DC has been upset by it countless times and has commented, I'm part of this family and I don't see them very much and sometimes they're horrible about you Mum, and our house and the baby and I hate being the odd one out and not having 'our' family's name and having their name instead.
My friend really regrets it, not least as she says (quite perceptively) that she really feels that DC would feel a bit less hostile about the father's side of the family if it wasn't such an issue, if DC had her name and it wasn't something his mother (especially!) could use as a way of marking her territory. DC sees it, feels uncomfortable with it, pulls away.
Changing the name will be the first thing they do at 16 apparently!
So. Your name, if he is already an ex it's an absolute no brainer.