I have both MX and motor boots and they are build different. It's my understanding that MX boots are not made to hold up as well as motor if you end up sliding on the road. MX boots seem to have a harder shell, more protection against crushing I'd say, like when you end up with your 4 wheeler on top of your leg, not that I would let that happen to me.

 

I was trying to get a pair before the driving part of the motorcycle safety cource, so I decided instead to get a pair online from ridersdiscount.com only to find the ones I ordered were out of stock and discontinued Now I have to wear my work boots for the weekend

 

More protection is always good, till you get something, just make sure you're not riding with shoe laces........ most dangerous thing out there.
Something else you may want to consider, some mx boots are so stiff that if you get off you bike to walk into McDonalds or the mall or someplace, you'll be walking like you have lead in your shoes, street bike boots loosen up after they're broke in, some mx boots are forever stiff......

 

It depends what kinda riding your doing. I just bought some new Joe Rocket ballistic boots, cause im planning on doing some long distance riding this summer. There the same size as a hiking boot, but there so stiff, their imposible to walk far in. But they offer great protection of the ankle.
But if your just wanting something to putts around town in. Icon makes some more flexible style boots. But they have way less ankle protection.
So it's a trade off.

The problem I see with mx boots is there made to keep your legs straight for hard landings, and to you don't twist your ankle in the mud etc. So i can see them being really uncomfortable while riding. And i think the extra steel will make it hard to feel the shifter