Which is better for a kindergartner?

The Mainstays. At 14 oz it is lighter and smaller, and the one-button lid is easier for small hands to open. The TAL is better once your child can manage a bigger, heavier bottle.

Which keeps water cold longer?

The TAL. It is rated to hold cold for up to 40 hours. Mainstays states double-wall insulation but gives no cold-hold time.

Which is easier to clean?

The Mainstays, which is top-rack dishwasher safe. TAL does not state dishwasher safety, so plan on hand washing the Ranger.

Is the TAL worth stepping up to?

Yes if capacity and cold retention matter most. You get a much larger bottle, a non-slip base, and a longer cold rating, at the cost of size and weight.

Will either fit in a lunch bag?

The Mainstays 14 oz bottle fits lunch bags and cup holders. The TAL is larger and may not fit a small lunch bag or standard cup holder.

Which has better owner feedback?

Both score well. The TAL sits at 4.7 from 694 ratings, the Mainstays at 4.6 from 63, so the TAL's score rests on far more reviews.

TAL Ranger Kids Water Bottle 18oz vs Mainstays Kids 14 oz Insulated Water Bottle

Buy the TAL Ranger for an older kid who drinks a lot and needs a bottle that survives a full school day. It holds far more, keeps drinks cold for up to 40 hours, and has a silicone non-slip base plus a carry handle. The Mainstays is the better pick for a younger child or a small lunch bag: it is lighter, smaller, top-rack dishwasher safe, and its one-button auto-chug lid is easier for small hands. Owners rate both well, though the TAL has far more ratings behind its score.

Choose the TAL Ranger if your child drinks through the day, needs a big bottle for school or sports, and you want the longest cold retention and a non-slip base.
Choose the Mainstays Kids bottle if you need a compact bottle that fits a lunch bag or cup holder, goes in the dishwasher, and opens one-handed.

Prices

TAL Ranger Kids Water Bottle 18oz: $7.60 at Walmart (checked today)