Tough call~ take my advice with a grain of salt or whatever, but I made it a personal choice to never date anyone from work, and also never any of my friends. There's a reason why you two were only friends and never got together for such a long time. It may work out in the movies and may sound like a good idea right now, but rarely does it ever end well. When it goes sour, you lose both the new girl and the friend. If you stick with the new girl, you'll always have that friend
It may seem like you're "deep" with the first girl since you've known her longer, but in a short amount of time with the new girl, it seems like you've grown together much faster.