Can you go wrong with either one?
That was a similar choice I was faced with but I ended up with the challenger. I've got two pre-teen boys as well. For us this ended up being a good choice as I've found not only are we bringing friends of the boys but often their parents as well. I'm not as familiar with the interior space of the Utopia, but I know my boat has plenty of room for a two families of four. The challenger also has the portable table which is nice come lunch time.
There are other considerations though. Where do you plan on storing it? My boat on the trailer is 29' long. I don't have the swing away tongue because my trailer has the hydraulic actuated disc brakes. Also the wake board tower doesn't disassemble that easily so it's something like 12' tall. It won't fit in my garage and is spending the winter at my lake lot. I think your family will have fun on either one but if you plan on regularly having 6 - 8 people with you, I would seriously consider the larger boat.
One other word of advice after reading the posts on here, ensure your dealer does a water test with the boat. My dealer not only did this but the salesman spent an hour with us on the lake they were close to at the time of delivery to show us the ins/outs of the machine. That was worth quite a bit and also ensures that hoses/clamps etc are all good and the boat is water worthy. Have read some horry stories here about guys having some misfortune with brand new boats. Luckily I've been able to just throw in fuel, flip the start switches and go. Any other questions about the 230 challenger, let me know.