Well not to point out the obvious, the big difference between Monitor Audio and Polk Audio is that the name is slightly different. You'll notice that Monitor Audio has the word "Monitor" in it. This is confusing to some because Polk Audio has a series of products named "Monitor". Oddly, Monitor Audio does NOT have a series called "Polk". However, this is a minor quibble I'd not concern yourself with.
Polk Audio is part of a multinational conglomerate that owns many brands, many of which specifically deal with audio equipment. Audio conglomerates are ruling the roost it seems these days. Funny thing is when you go to Best Buy and look at speakers the most independent designed product on the shelf is probably Sony. Black is now white and up is down.
Monitor Audio is still it's own company (for now) and is headquartered in the UK. A many of their products are still built in country with a few being outsourced to China (pretty standard stuff these days). With Monitor Audio you'll get a bit more pride in ownership. The fit and finish of their products is exceptional.
Polk Audio is mass market...stuff, aside from their upper tier LSI/m series, they're not a product I give much consideration. This, is just my opinion.