I would definitely invest in the double stroller. My son was 2.5 when my daughter was born.
For our active lifestyle, we bought the Bob Revolution Duallie. LOVE THAT stroller!
At 2.5 my son would take off running from me whenever we were outside the house, so I mainly used the Bob to keep him contained.
Now my kids are almost 4 and 19 months. For the most part, I can take both kids to the mall and oher places using our single Bob Revolution stroller. If one gets tired, we switch out whose sitting in the stroller. Or will have DS just sit on the foot part of the stroller. But we still use our Duallie for long trips to places like amusement parks, long jogs/runs, etc.
So we used our Duallie everyday for about 1.5 years without fail.
In deciding what double stroller to get, I would definitely take into consideration your lifestyle. For us, the Graco stroller never worked. The wheels were too small. Our Bob strollers roll over EVERYTHING! We have taken them camping, to the beach, amusement parks, hiking and they roll over EVERYTHING! I would also go to a store like USA Baby and test drive strollers before buying. If you do decide to get a Bob stroller, definitely get it through REI.com and have them free ship it to your local store. About twice a year they will have them on sale, and that' the best price you can find for a brand new one. You can also try craigslist. I have seen some nice used and even a few new in box ones on sale there too.
My friend who has 3 kids has had an array of double strollers. Peg Perego Duette, Sit and Stand, Combi side by side, and Graco double stroller. She hated the Duette and the Graco ones. She uses her Sit and Stand sometimes and the Combi mostly. But she really loves my Revolution Duallie! And more often than not, when we go out together, at least one of her kids ends up in my stroller :) so we just grab whatever stroller is closest to us and start pushing when we are out and about, no matter whose kids are in what stroller. She always prefers to push the Duallie over whatever stroller she brought with her.