How much riding do you foresee yourself doing? If your just going to go on short rides with the family, then the Crosstrail disc should be fine. The really big differences between the Disc and Sport disc as I see it: rear derailleur, gearing and the brakes. The Sport disc is better, but if your only going to put like 15 miles a week or so, the regular Crosstrail disc should be just fine.
Comparison:
Altus vs Alivio - Alivio in my opinion is a bit smoother to shift and stronger. Altus will need to be adjusted more often, but has worked well so far on my Wife's bike
Gearing: The Sport has more gear ranges that helps better with going up a hill. I've also noticed it helps more with going slower if you have a child that is slow on her bike (like my daughter).
Brakes, My son has these mechanical ones and they work well. I have the hydraulic and really the only major usage difference is how easy it's to pull the brake levers. But I have also noticed that cleaning and adjusting my hydraulic brakes are a whole lot easier than the mechanical. Taking the pads out of the mechanical is a bit of pain since Tektro uses a pin that needs to be bent rather than a screw.
If your willing to wait a few weeks, I would choose the Sport. But that's my preference knowing that I've been riding upwards of 200 miles a month now. IF your riding just like 40 or 50 miles a month. Then save your money and get the regular Crosstrail disc.
BUT, this is your decision. A happy rider is one that will keep riding!