I can't give you 1.7 vs 3.7i but I can give you 1.6 vs 3.6
both of which I own / did own.
The difference between the quasi-ribbon 1.6 & ribbon'd 3.6 is not subtle. It's readily apparent in resolution, enhanced soundstage "depth" cues, better imaging, plus there's lower bass extension.
I have a particular Opus 3 reference recording that was used in auditioning 90's vintage Soundlab e-stats that had tremendous apparent depth. with 1.6's, the sense of depth was compressed for lack of better word while the 3.6's brought it back to what I recalled from the Soundlabs. Speakers in the same room, same position, same amp, same player & electronics.
Now, at my local Maggie dealer, I have noticed in several demos that this recording for whatever reason doesn't give quite the same illusion of depth in the middle with 3.7 & 3.7i's that I get with 3.6's. I have no explanation nor does the dealer & Wendell Diller @ Magnepan absolutely won't comment on design differences between 3.6 vs 3.7 and 3.7 vs 3.7i. But I suspicion they've made some crossover as well as the driver changes between the 3.6 & 3.7i to get the better integration. Of course, his demo room is completely different with different amps & player. While I've listened to both 3.7 & 3.7i at my dealer, they were a year apart in 2 different rooms so I can't tell you if difference it's subtle or big.
But the bottom line is if you have the budget for the 3.7i, don't hesitate to get those over the 1.7. If you go for the 1.7's, you may find yourself in the position I was - trading them in for the 3.7i in several yrs. Might as well save yourself time and get the 3.7i to begin with
While the 1.7 is the sweet spot in the line & fantastic speaker for the money, the 3.7i is just a better speaker.
I completely agree with these comments from DonH50:
"None of the listening I have done since then has changed my opinion that the 3.x is a significant step up from the 1.x series but it is a very personal thing."