Mini-Zs handle much better, have better electronics and controller, better design, nicer more realistic bodies, much lighter, faster out of the box, longer range, but the biggest factor is they are much more expensive.
Xmods are more of a toy, where Mini-Zs are hobby grade RCs. If you plan on racing, or would just like a better handling car with a much nicer selection of more realistic looking bodies then the mini-Z is the way to go.
With that being said, a brand new, stock AM mr-02 (2wd miniZ) is about 3 times the price of a brand new Xmod(non sale price)
Here is a link to some mini-Zs so you can check out the prices.............
http://www.atomicmods.com/Categories...eady-Sets.aspx
I think the Mini-Z is worth every penny, but I like to race and have a small track of my own.
I still like Xmods as well, I usually just change the PCB(electronics/circuit board) and put in the PCB from a Mini-Z so I can drive them with a hobby grade controller.
You could always look on ebay for used Mini-Zs, but as with any used items that you buy on ebay, you never know what your going to get.
If you do eventually plan on getting a Mini-Z, I would suggest a MR-02 or MR-15(both are 2wd), or if you want an AWD car then there is the MA-010 which costs a bit more and is a bit harder to set up.
The car that I just got is an MR-01 which is the original model Mini-Z and is no longer in production. It's still a great car and handles awesome but since it's obsolete parts are a bit harder to come by, so I'd stay away from it if I were you, unless of course you get it at a great price like I did.
Since you have 3 sons and a few cars already you might want to buy or build a track if you don't already have one, then if you do end up buying a mini-Z it will really "shine" since a Mini-Z is built for racing, and you'll be driving circles around the rest of the xmods.