I still have both. The Genius 10 is boa-like (2 dials), and my old Genius 5 are two velcro straps and a ratchet strap.
Caveat: I use the mega width, so kind of opposite of you.

I'm not sure if this is just the age of the Genius 5s, but the biggest difference I've noticed is that the Genius 10 "leather" is a lot stiffer than the Genius 5. This hasn't bothered me: the toe box is wide enough that the stiffer leather doesn't bother my toes. Nor the rest of my feet.

The wires feel more secure than the velcro. Again, possibly due to the age of the velcro. But, the wires around the upper seem to "hug" my foot when tightened, rather than just pulling the top closer together. So, the connection between my foot and the shoe feels tighter. This can be better (power transfer feels better) and worse (Genius 5 feels more slipper-like), depending on whether your hammering or just seeing the sights.

On the bike, it's harder to loosen the shoes with the boa wires. It's not hard, just more fiddly. I generally have to loosen using the buttons on the side of the dial, then flip the dial tab thingy up so I can retighten. OTOH, there are only two dials, whereas the Genius 5s had two straps and a ratchet. Maybe it's just me getting used to it, but it's been a couple of years and for me the wires are still more difficult to loosen during a ride. Tightening is also a bit harder, you have to flip up the tab, which is pretty hard to do during the winter with full-finger (or, even worse, insulated) gloves.

When gearing up, I usually put my shoes on, tighten them, then walk around a bit getting the rest of my gear ready, then adjust the tightness once the shoe is warmed up a bit. Sometimes I forget to tighten them, which wouldn't be a problem except that I usually leave the tabs on the dials flipped up while I'm walking around, and I sometimes forget to flip them down. This is more a me-problem than a problem with the shoe.

I'd say the Genius 10s are just a little bit warmer, but not excessively so. I don't go on a hot summer ride and think "I wish I had my old Genius 5s" because my feet are so much hotter.

I haven't noticed a difference in sole flex, but I'm just a weekend warrior so I'm not capable of putting out enough power to evaluate the limits of the sole flex of either shoe.

Overall, I'd say the first point is the biggest: the old Genius 5s feel softer and more like an old friend, the 10s are stiffer and feel "racier", and the closure fits around my foot a bit better.