The verdict
If you already have visitors but your forms bring thin leads, I'd pick Outgrow, for an ROI calculator or a readiness assessment on a high-intent page.
If you capture leads but follow up by hand, or not at all, I'd pick Brevo. A few automated emails and a text will earn more than a better form.
They work well in sequence. Outgrow lists connections to HubSpot, ActiveCampaign and Mailchimp, so check how its leads would reach Brevo before you buy. For plain contact forms, a cheaper form tool does the job and you need neither.