It depends upon whether you want a surround speaker that is capable of producing a diffuse soundfield or not. Personally, I'm not a fan of any speakers that produce a diffuse soundfield. Some people who have systems that are predominately for HT prefer them for their surrounds because they seemingly provide a more enveloping surround soundfield. If you plan to do any multichannel music listening, then a non-diffuse speaker is what is preferred.
There are basically 2 "camps": 1)people who are pretty adamantly against surround speakers that can produce a diffuse soundfield and 2) people who are pretty adamantly in favor of surround speakers that can produce a diffuse soundfield.
The benefit or attraction of the "fx" speaker over the "i" speaker is that it is actually capable of being switched between a "Solid" (non-diffuse) mode and a "Diffuse" mode. So you get both options, non-diffuse and diffuse, depending upon your room and placement characteristics and your own personal taste for whatever it is you are watching (hearing).
However, in reading about each speaker, it seems that the "i" speaker may have some other adjustability features (tweeter level switch, wall distance toggle switch) that the "fx" speaker doesn't have.
So, if a diffuse surround soundfield is important to you, then the "fx" speaker would probably be the one to buy. However, if this is not important to you and you'd prefer a more "solid" and discreet surround speaker sound, then the "i" should fit the bill.
BTW, the Polk site seems to indicate that these speakers have identical costs. I guess you're getting a better deal on the "i"s.