The real question is who runs retention
Rejoiner makes more sense when you want help. Klaviyo makes more sense when you want a platform your team can operate directly.
That is the core difference. Both can send ecommerce lifecycle messages. The better choice depends on whether you need execution support or tooling depth.
Where Rejoiner wins
Rejoiner wins when the retention program needs strategic help. If you want a partner involved in the campaigns, not just software, it fits.
Where Klaviyo wins
Klaviyo wins when your team wants to build and improve the program itself. It has more platform depth and a larger ecommerce ecosystem.
Where Sequenzy fits
Sequenzy fits when retention does not need to be complicated. If email is enough, keep it email-first.