It all depends on what functionality you are using. There is no easy answer, weigh it up against your specific situation.

in v8, attributes are very different and the update from 5.7 has been flakey. If you have not got too far and use attributes a lot, better to start again with a clean install of v8.

Forms in v8 use Express and are very different to 5.7. 5.7 forms are supported in v8 (legacy form), but perhaps you would be better with Express forms for the future, so again better to start again in v8.

Express forms are very complicated to custom-style vs 5.7 legacy forms.

v8 has been buggy on support for non .jpg files. If you do a lot with .svg, better to wait.

v8 contains lots of new code and some extensive re-work of 5.7 code. Some of that is not as mature as 5.7.5.13 and has led to the kind of problems you see on these forums.

Class namespacing is again different in v8. There is backward compatibility support for 5.7 code, but perhaps not 100%. Consider that if you have custom packages.

If you have already created lots of pages, perhaps starting again in v8 would be too much work and an upgrade is preferable.

If the site is primarily a brochure, v8 is not that different to 5.7 and may be more reliable.

Check the support of marketplace packages before updating.

Whatever you decide, take a files and database backup. That way you can always get back if it goes badly wrong. Ideally practice and test the update on a clone.

Thank you. I have some things to think about.