The second one looks like the one I got from rock auto although I do not have cruise control. Cruise control does look like it was an option for the car though. Does one sensor control the speedometer and cruise control via the ECM? I decided to get the aftermarket one I did through rock auto since it was only $21 and they only had three left. Without having performed the tests Gus mentioned, I'm leaning towards the cluster being bad. The driveability of the car seems fine and I learned the speed sensor also sends inputs for the torque converter clutch so I don't think it'* the ECM (I'd really hate having to replace that not to mention all the trouble locating a good one).

After looking at both of the ones I posted links to again, they both look the same, I think the one that had the wiring connector part of it threw me off.
Unfortunately I am not sure the answer to that question.
As for getting aftermarket sensors, I have never had good results from doing so, I got an aftermarket ECT before, and the coolant temp reported to the dash gauge was off by over 30F, with another sensor the gauge went all the way to the red, as soon as I put in an AC Delco ECT the problem went away.
Same with the oil pressure sensor, I replaced with a non OE brand sensor, and had readings that were off, my red idiot light often came on, and since the sensor can act as a fuel inertia shutoff, the car would occasionally refuse to start, as soon as I out in an OE AC Delco it was fine once again.

Some may have good results from non OE sensors, but for me, and a few others that is never the case.
It is always worth it in my opinion to spend a little more for that OE sensor, in the end you have a lot less headache, which makes paying a little more well worth it.

If I knew what the ohms values from dash to gauge were supposed to be for the cars speeds I'd say measure the resistance from that wire to the dash gauge to see if the ohms are off, and if so by how much, then I'd say get a certain value resistor, and temp put it in place of where the wire goes to the gauge, if that made the dash gauge display correctly, I'd then put money on it being the sensor used.
Of course in your case you said the gauge was slowly failing over a good period of time, so it may well be the gauge, but I am leaning more towards it being the sensor used, of course I'm only guessinghere, best bet would be to do as Gus suggested if somehow possible, that or send the aftermarket sensor back an opt for the OE AC Delco Sensor.
I'll go back over this post in a bit, and see if I made sense, just had my coffee, and it has not kicked in enough yet.

I'll try to see if I can find a list of the ohms the gauge is supposed to send out to the gauge in relation to the speed, but if I am not able to maybe one of the others can be more helpful.