The 170s are a better speaker. They play like a much larger speaker compared to the HTM-200. They can play lower than the 200s as well. Since they can often be had for same price as the 200s I'd never recommend the 200s over them unless you need the small size of the 200 or you plan on mounting the speakers flat on the wall. And even then I think you can plug the port on the 170 and it may still be a better speaker than the 200.
Would I pay to upgrade to the 170s if I didn't have a use for the 200s? That's a harder decision. I'm using the 200s for front wide speakers right now, but I think I'm going to pick up another pair of 170s for wides and use the 200s for heights.
My setup will be:
(2) 340 L/R
(1) 340 C
(2) 170 Sides
(2) 170 Wides
(2) 200 Heights
I have a open back wall and no place to put dedicated rear speakers unless I mount them near the ceiling. So I will run this 9.2 setup.
Driven with a Denon AVR-X4000 & Emotiva XPA-5.
When I was looking to add surrounds to my three 340SEs, the 170s and 200s were at equivalent prices. I chose the 200s because of their smaller size and lighter weight. That made wall mounting easier. I do agree that the 170s play lower, however, for surrounds, that is not as important as it would be for the front three.