The Mizuno belts are BY FAR the best bang for your buck in terms of belts. When you first get it it feels a little flimsy but trust me, once it breaks in a bit it is perfect. Its my favorite belt and I will most likely get a mizuno whenever I get my black belt.
The Isami is also a fantastic belt though. Now that it's broken in a bit I love it. It is the perfect width and thickness and fits comfortably. It comes off if you tie it loosely, and it stays on if you really tighten it. It was the almost the perfect size (an inch short maybe) so I worried about washing it but somehow it seems to have gotten a bit longer after washing and now its the perfect size too. So no worries about shrinkage.
If IBJJF didn't have the new rule about requiring a stripe bar on your belt, I would have stuck with mizuno. Since you have to have a stripe bar now, I wouldn't hesitate to recommend Isami at all (or save $30 and get a mizuno and sew your own stripe bar on
)