You can find lots of decent summaries online from the various pharmacies that prescribe both eg https://onlinedoctor.asda.com/uk/wegovy-vs-mounjaro-vs-ozempic.html
In a nutshell, both are GLP1 agonists, that means they mimic the natural hormone GLP1, which affects how full you feel and how hungry you feel between meals. Mounjaro has another action, where Wegovy doesn't, in that it also mimics a hormone called GIP. GIP affects your blood sugar levels and how your body stores and uses fat. Mounjaro is therefore more effective at producing weight loss than Wegovy, although both produce significant weight loss (16% of starting weight on average for Wegovy, compared to 22% of starting weight on average for Mounjaro).
Mounjaro is more expensive than Wegovy. It also has fewer side effects reported, on average, and they tend to be lesser in severity.
When I started, Mounjaro was cheaper, so it was an easy decision to make. I also had a lot of weight to lose, so wanted the higher rate of weight loss. If I were starting now, I probably would still choose Mounjaro, but that's because I am very fortunate not to have an issue with the cost, even though it's much more expensive than when I started. If I was more concerned about cost, I would give Wegovy a try and see how I got on with it.