I own one of each. Since I'm not real in love with either one, I think I can give an objective opinion.
The vac filler is just COOL! I love filling vac pens. But, because the pen holds so much ink, it takes a while before I get that joy again. The mechanism also makes it more difficult to clean the pen. The rubber bits are a bit more specialty and could be hard to replace in the future if something happens to the company. So, if you're making a choice, don't make it based on the filling mechanism. You don't experience this joy often enough for it to be important.
However, I find that the vac700 just has a more solid, quality feel to it. It is easy to write with. It's attractive. It is comfortable to hold. You do have to unscrew the top when you write, but that doesn't bother me.
Neither pen is particularly nice posted.
But, I will say that the differences are a matter of degree. If you like TWSBI, these pens are comparable, and I think you would be happy with either one. TWSBI has done a lot of work to create good pens, and they are steadily improving in quality. It may not be my personal favorite brand, but I have no problem recommending the brand.