Simon Hamp

As a Laravel developer, I'd have to say go with Laravel. Although you can move away from PHP, it means any of that legacy code will still need to be completely ported to Java or whatever other language you use. Whereas, staying with PHP, you may find it easier to re-use/adapt that existing code.

Of course, if you're going for a full rewrite, then that might not be a useful consideration. However, even a full rewrite will introduce a lot of changes. When having to do this kind of thing, I recommend changing as few things as possible at once. So staying in the same language while upgrading may keep some pain points down.

Being a Laravel dev (I've used ASP.net too), I would hands-down recommend Laravel. It's convenient to build on, easy to deploy, and there are plenty of skilled Laravel engineers in almost every country. So if you ever want to bring development 'in-house' or work intermittently with freelancers after you've finished with the agency you're working with, you'll find that quite easy wherever you are.

As has been mentioned, Laravel is very ergonomic for developers and it has an incredible community (mostly filled with really positive people) and ecosystem (lots of ready-to-use open source packages to augment your application with new features).

Development costs for the two I'd expect to be about the same, but possibly the agencies using .net will charge a little more as it has historically commanded a slightly higher price point, but I'm not sure how true that still is.

Both will get the job done perfectly well. Laravel would be my choice.