It depends on a lot of things.
If the children are small and demanding and don't nap, plus don't sleep through but you do all night wakings because you are a sahp, and all housework and all grocery shopping and all cooking and all laundry (all with toddlers in tow, especially multiple toddlers or children of a range of ages all with different schedules to juggle) because you are a sahp, then being a sahp can be more tiring than going to work.
It also depends on the work - sitting at a desk all day is brain tiring and might give you back ache but is nothing like nursing or care work or manual labour in terms of how tiring it is.
Commute is also relevant - how long and what mode of transport etc.
It depends on 101 things, but I have worked full time in an office job before children (50-60 hour weeks) whilst doing a masters degree (over 2 years instead of one) in my 20s, worked full time as a secondary school teacher pre children and with one child, been a child minder, a teacher and a care worker all post children, and been a sahm to various numbers of children, finally to 3, before going back to work again.
Having done all those different combinations I would say the absolute most tired I have ever been is when DC3 was a sleep refusing small toddler still young enough to be quite feral who climbed everything and had to be watched constantly, woke multiple times a night, and had older siblings who had to be got to and from school and preschool on a schedule so that I had to haul us all out of the house before 8am and couldn't just snooze on the sofa while older kids played/ watch beebies if the youngest happened to have finally fallen asleep at 5am and be inclined to wake naturally at 8:30am...
On the other hand being a sahm to two preschoolers who both slept through, and one of whom napped in the afternoon for a couple of hours, was one of the easiest and happiest times of my life - I felt for a brief window of a couple of years that I was doing everything properly and was well rested and my life really was easy even though I still did all the domestic work.
Working can be easier or harder, more or less tiring than being a sahm - it depends and is a how long is a piece of string question.