In my view, it's vital to choose a bike that fits and is comfortable to ride, otherwise those will be the primary reasons why it gets abandoned in the garage. Feeling safe (e.g., trusting the brakes) is another important factor for a less experienced cyclist. Being able to look at bikes in a shop and try out a few of them might be worth a few more bucks. And it's worth looking at the range of options other than mountain bikes, such as hybrids and city bikes.

I'd suggest making sure any bike you buy is capable of accommodating a kickstand. My rationale is that if you ever want to try out a kiddie trailer, a kickstand makes it considerably easier to manage. Also, these days, a bike with puncture-resistant tires will make for a better overall experience.

Something I've noticed in threads about box-store bikes is that the success stories tend to involve doing some adjustment and repairs immediately after purchase. You or your spouse need to be able and willing to take care of those things. A bike from a regular bike shop is more likely to have had those adjustments made before you buy the bike.

I'm quite happy with my DiamondBack bike, purchased from REI last year.