For me (and please correct me if I'm wrong, Instant team) the largest difference and a major source of frustration in trying Instant is that you don't have a backend. You get a real-time database with RLS and a sync engine. You don't get to run business logic. You don't get to write queries by name and gain the ability to migrate the schema as long as the query shapes match.

I would say:

We both offer a real-time queries out of the box. I am not 100% sure, I but think Convex also set up a multi-tenant database; so they can offer a good number of free projects well.

The way I would differentiate Instant:

With Convex you write your queries as Javascript functions. This means you have to do joins for example imperatively. With Instant, you can write queries declaratively.

As of today Convex doesn't work offline, and you have to write optimistic updates manually. Instant can run offline and comes with optimistic updates out of the box.

Both Convex and Instant support files out of the box. But with Instant you can write CASCADE delete rules, and you also get other services, like presence and streams.