The primary difference is the LS (Leading Screw) allows microsopic (1/64") fine-tuning for either advanced dovetail work or to micro-adjust a setting. I have the TS-LS with WonderFence setup. While I've use the leading-screw feature in situations where I've needed the micro-adjust, this hasn't been often-enough that I would justify the $180 that would be required to change-out the setup - this is my values stream speaking, and I can't make that comment from your perspective. Also, if I did advanced dovetails, I'd probably go with a Leigh jig, rather than use the Incra - I've been there, and it takes a lot of practice to get proficient with Incra dovetailing.
Regarding the LS17 vs LS25, there are two considerations - the LS25 is $30 more, from Amazon; and the Router Table itself has to be large enough to accommodate the extra spacing (assuming you don't build an aux table to support the Positioner base). From a purely practical view, I'm not sure the extra 8" capacity buys a lot.
I hope this helps.