I never open my door, so I don't know if I'd ever bother to spend time or money replacing it. You may use your door in which case, I'd go for the stainless steel one if you don't mind spending the money.
As for the therm - there is no way I'd try to replicate the factory therm from the newer model. I have never seen the new therm so I can't say for sure, but my guess is that it's not high end and possibly even cheap. If you're gonna go cheap, what's the point? If I was going to spend time and money, I'd get a much nicer thermometer and I'd probably go with one with a longer stem. For me (on my old wsm), I just drilled a whole through the top vent rivet and put a cheap thermometer through it. Quick, easy and cheap. More importantly, it works just fine.
I guess I just don't see much on the new wsm that I feel I need that I don't have on my old version and therefore, would not "upgrade". I don't use water, so the new water pan doesn't do anything for me either.
I think if I wanted to upgrade and spend some money on this hobby, I'd look into the automatic temp controllers. That would give me something I don't currently have.