Never ever get angry at her or be douchey about it. This is a very hard time for her.
Nonetheless, I believe you should stand your position firmly. It really comes down to how it affects her body. Abortions are from what little I gather, are nothing compared to an actual pregnancy - vaginal or c-section. At least this is what I assume, it's just a pill or a non-intrusive procedure.
If it is physically better for her, I don't see why you shouldn't stand your ground. Especially if you are already not sure about the relationship and not sure about having a kid who you might fuck up for the rest of his/her life by not being there enough or having the financial means to support it.
Any moral arguments don't work also because the fetus does not feel or think as far as I know (despite religious people claiming so). Also I've never read a study that suggested that having a child makes couples happier, as a matter of fact, I believe I have only read of the opposite.
They may also be incredibly intelligent and be the next rich and famous Bill Gates and make you eternally carefree or they could have severe physical or mental disabilities, and destroy your carefree existence in a lot of ways.
For me this is a deal-breaker, and the chances are low, but I still couldn't take the chance personally, and wrong as that may sound to people.
Anyway, like I said, it comes down to you having a strong position on this, backed up with the fact that you aren't ready, you likely will not be any happier, her body will go through some shit if she does and the child may suffer in the long run. Stand firm, but don't be a douche about it. Also don't forget to do a DNA test.