They will be just fine, hold up great! My friend's kids are 6 and 4 and she's had the City Mini Double since the 2nd was born and it has been great and the wheels are great. They will definitely last you. I have the GT double and LOVE the wheels on that one, we use the stroller fairly often since LO #2 was born (she is almost 4 months) and it's held up great. Also, when you go to Disney, the #1 stroller that is rented by families are the City Mini and City Mini Double, so they must hold up great for them to be used almost every day and walking forever in those parks!
I don't have a double, but I love my single city mini! We have only had a minor issue with the front wheel on our City mini...I started to drag instead of swivel, like it should. I tried hosing it out in case something got stuck, but it didn't work. I contacted City Mini and they sent a replacement ASAP and now it's back to working like a charm!
I have the GT, but haven't used it yet. Similar to pp, my friend has the City Mini and had some issues with the wheel sticking and contacted Baby Jogger and got a replacement wheel very quickly.
Re: If you own a city mini come in!
We hate the wheels, we use the stroller occassionally for travel, and it's gotten stuck on gravel, cobblestone, grass and sand. I'd get the GT wheels.