Hi D.,
One of the problems, I found with the side by side strollers when one child is significantly heavier than the other is the weight imbalance throws off the front end making it a little awkward to push.
It depends on what you will use the stroller for. If you are seriously into jogging for excercise and will be taking your little one with you than I say invest in the double jogger. Your 2 1/2 year old will be able to ride until about age 5.
If you simply need a stroller for the mall. I say go for the lightest weight double stroller with the smallest profile to make it easier to manipulate around the tight spaces in stores and easier to get in and out of your car.
As much as your 2 1/2 year old likes to walk there will be times when he is tired or uncooperative when you will want the convenience of a seat for him. I've seen some new strollers that have a sling like seat which is behind and somewhat under the main seat. This seat can be used for a newborn while the bigger child sits in the main seat. As the baby grows he/she will graduate to the main seat and the older child will walk more. Then the sling could be used for storage. I must say I found it difficult to stay home all day with two preschoolers (my girls were 21 months apart) especially in the winter. So we frequently took trips to the mall to have lunch or just to walk around when the weather was bad. This kind of stroller seems like a nice compromise. That way you won't be stuck with a bulky stroller as your older child grows up. And let me tell you, my girls are 7 and 9 now and there are still times when I wish I had a stroller to carry the coats, bags etc.
This stroller I am referring to, (sorry I'm out of the carriage years and I'm not sure of the name) also has front swivel wheels making it more maneuverable and rubber tires (though not as large as jogger) making it a good hybrid for most surfaces indoors and out.
I can say I purchased the sit to stand and found it bulky and difficult to maneuver. Best Wishes.
J. L.