Which is better for someone with no Kobalt batteries?

The XTR Starter Kit. It includes a battery and 45W charger, so the saw works as soon as it arrives. The bare saw needs both before it will run.

Do both saws use the same blade size?

Yes. Both are 6-1/2-in cordless circular saws on the Kobalt 24V platform, so blades and batteries are shared between them.

Is the bare saw worth stepping up to if I already have batteries?

Yes, if you want the published bevel range, electric brake and LED light without paying for a battery you already own. The kit makes more sense only when you need the battery and charger.

Which one lasts longer?

Both use brushless motors, which outlast brushed motors. Battery life depends on the pack you use; the kit's single battery will need recharging during long jobs, while the bare saw lets you swap among packs you already own.

Can I use the kit's battery in other Kobalt 24V tools?

Yes. The kit battery is a standard Kobalt 24V lithium-ion pack and fits other Kobalt 24V tools, and the bare saw accepts the same packs.

Does either saw have a laser guide?

No. The bare saw lists no laser guide, and the kit listing does not mention one. Use the notch and edge guide for straight cuts.

Kobalt 24V Circular Saw vs Kobalt 24V XTR Brushless Circular Saw Kit

Both are Kobalt 24V brushless circular saws with a 6-1/2-in blade, so cutting performance is close. The real split is the kit: the XTR Starter Kit includes a battery and 45W charger, while the bare-tool saw does not. Buy the bare tool if you already own Kobalt 24V batteries; buy the kit if you are starting from nothing. Owners of the bare saw rate it highly for its compact size and electric brake, but note the extra cost of batteries and charger.

Choose the Kobalt 24V Circular Saw (bare tool) if you already own Kobalt 24V batteries and a charger and want the saw on its own.
Choose the Kobalt 24V XTR Brushless Circular Saw Starter Kit if you have no Kobalt 24V batteries yet and want to cut the same day you unbox it.

Prices

Kobalt 24V Circular Saw: $75.25 at ISCC Community Center Project (checked today)