Congratulations! I am a mom of 15 month old b/g twins and I LOVE my Mountain Buggy side-by-side jogging stroller. Of course I don't actually jog with it, but the wheels are large and it can run over virtually any surface. This brand is expensive, but a good group gift and well worth it. You can also find a lot of used strollers on craigslist, etc. Since it is such a well made stroller, they typically wear well and last long.
It seems a lot of the posters are doing a lot of shopping, therefore preferring the front-to-back. I, however, try to stay out of stores as much as possible (hubby gets a list to grocery shop on his lunch break) and use my stroller daily for walks to the park, quick trips to CVS, etc. (I live in Oak Park, a very walking-friendly town). I love the side by side because you can see both of them at the same time and now that they're older and the seats are adjusted higher, they hold hands and pass toys back and forth.
I think you need to analyze how you will use the stroller and where you will go. The front-to-back doesn't maneuver as well as side-to-side, but does get through areas easier. That said, my stroller gets through all doors and bigger aisles (no problem in Jewel, CVS) and there has to be at least one lane in all check-outs that is handicapped accessible, so the stroller fits through that too.
Lastly, I got a well-worn used double umbrella stroller (craigslist again) that I toss in the back of the car and use to get kids in-and-out of car for quick trips. Also worked great at the airport and on vacation. I paid $8! It isn't pretty, but I don't have to worry about it getting banged around and it is super light and easy to open/close. This will be helpful as before they walk, it is next to impossible to get them in and out of car by yourself, especially during winter.
Best wishes!