The difference between the two
Local Services Ads sit at the very top, show a verified badge, and charge you per lead rather than per click. Getting in takes proof the business is registered, your license, your insurance and a background check run for Google by somebody else.
Ordinary Google Ads sit below those and above the map. You pay when somebody clicks, whether or not they call, and you can run them for any trade with no verification at all. That freedom is why they're easier to waste money on.
Check the list first
Google publishes the categories eligible for Local Services Ads in the United States, and it's specific. Roofing, plumbing, electrical, HVAC, landscaping, garage doors, remodeling and general contracting are on it.
Paving, asphalt, sealcoating, driveway work, concrete and masonry are not. If that's your trade, the per-lead ads aren't an option today, and the answer is ordinary ads or nothing.
Don't sign up under a nearby category to get in the door. Google's rules want the category to match the business, the leads you'd buy would be for work you don't do, and a mismatch puts the account at risk.
What ads can't do
They don't move the free results or the map. Google says in writing that there's no way to request or pay for a better local ranking, so an ad buys you the space above the list and nothing inside it.
They also stop the day you stop. That's the whole difference from the pages: an ad is rent and a page is something you keep. Both can be worth it, and they're worth it for different reasons.
And neither one writes the page somebody lands on. An ad sends a stranger to whatever is there, so a click bought for a job in a town, landing on a home page about nine services, is a click you paid for and lost at the door.
When ads make sense for a contractor
When you need work this month and the pages haven't had time to do anything yet, or when a season is short and you can't wait. Set a budget you can lose, aim at your best two or three jobs, and watch what a lead costs you.
What doesn't make sense is running ads forever instead of building the pages, because the meter never stops. Ads while the pages come up is a plan. Ads instead of pages is a monthly bill with no end.