My kids are 13 months apart so I wanted a stroller that would last a long time. I got the baby jogger side by side stroller and am in love. I never use anything else. It was outrageously expensive. Sometimes at the doctor or something, I carry my daughter (8.5 mos) in a front carrier, and let my son (22 mos) walk/hold my hand. Total nightmare. I can't control him at all with her 20 pounds hanging onto me. Maybe your son will be better because he's older and can be rationalized with? But I think you're totally right to be worried about managing both when out and about!!
I used a snap n go and HATED it. Sure, it was convenient (actually perfect, in a mall setting) when I had JUST the baby, but it doesn't have good suspension, hits every bump and jolts the baby, and is hard to push while negotiating streets/curbs etc (not sure what you'll be doing). I definitely could not push it while carrying my son or holding his hand. But like I said, your son is older so maybe things will be smoother for you. The alternative is a backpack carrier for those desperate times, but 41 pounds is pretty hefty, and you'd have to wait until 6 months to put the newborn in it. I have an Ergo carrier (front, back, hip) and just love it for both kids. Good luck, and congratulations!