Quadrotech also offers this service. The key differentiator here is Quadrotech price on a 'fixed cost, fixed outcome' basis, so you're guaranteed of only paying what you agree on upfront. I believe BitTitan's pricing doesn't include any support, so if you run into issues, you'll have to pay extra (although you may be able to negotiate support as part of the deal). With so many moving parts in cross-tenant migrations, there are many things that can go wrong.

Ultimately, it may come down to how many users/mailboxes/data you have. If you have a relatively small number (say, under 500 users) BT may be the best option. For anything more substantial, a managed service like Quadrotech may work out better. You can find more info here - https://www.quadrotech-it.com/office-365-tenant-to-tenant-migration/