I am in a very similar position. I, recently, purchased a home built in 1916 that currently has a finished attic. After talking with contractors and designers, and especially a bank that does 203K Loans. I've decided to add large gables rather than an entire seacond floor.
One gable will be just a master bedroom and the other will be a larger bath and allow room for the new stairwell and a sitting room. This will still allow us to have two smaller bedrooms with the attic style ceilings at each end of the house. There were several reasons we decided to do this.
A big reason was price. Here in our area, it's costing $90 - $100 a square foot for new construction. That's simply more than I am willing to spend.
My best source for assisting with my decision was a bank that does 203K loans. They'll assess your costs vs. the financial rewards of the addition or remodel.
It's also very important to talk with a designer and have them take a look at your foundation. Your foundation's ability to accomidate an additional floor and attic can, often, make your decision for you.
I think that if you plan on spending 15 to 20 years in your home, any addition you do, within reason, is worth the time and money.
I hope that I was able to help.