OPNSense is a fork of pfSense. There is a lot of drama out there about the relationship between the two.
pfSense is a rock solid platform. Netgate offers several boxes with 10gig NICs and pfSense pre-installed.
I feel pfSense is ahead of the curve and OPNSense drags behind, pushing hacks into the mix to keep up. ARM is a good example of this.
pfSense also contributes a great deal back to FreeBSD - IE: ARMv6.
I've spoken to many businesses running pfSense, ranging from data-centers, enterprise businesses, small offices, personal use. Most of the feedback I've heard has been extremely positive.
TL;DR, pfSense is a rock-solid mature platform. Simple as that.