I'm not sure I understand this exactly.
based on what will work longer, that's anybody's guess. I would expect both displays to last longer than you need them to, unless there's a defect(which can't really be predicted).
based on what technology will become obsolete sooner, I think is kind of a silly question. on one hand, it's quite obvious that plasma will be discontinued before lcd is, so you could argue that the plasma will be 'obsolete' sooner. however, since the plasma still provides a better picture in many ways, you could also argue the lcd is already obsolete in comparison. either way, it's a ridiculous argument imo. so all you can really look at is what inputs it has, what it needs, and what sources you intend to use. the big difference here will be the 720p plasma vs a 1080p lcd. 1080p will be relevant, especially in the under 50" sizes, for many years to come. it offers the best possible resolution of the highest quality video source currently available(bluray). the other thing going for the lcd is it's 3inches larger. the smaller the screen the sooner you'll want to upgrade. at least that seems to be the trend. it sucks upgrading a perfectly functioning tv because you underbought.
imo, the difference between 120hz and 60hz is pretty minor except for how they handle 24fps movies. 120hz TV's obviously do that better since 120 is divisible by 24 evenly. no added/dropped frames necessary. beyond that though, I've never seen a difference with 30 or 60fps material.
in the end, neither tv is anywhere close to future proof. they are already considered 'old technology'. but since they both have hdmi inputs, they should technically be useable for exactly the same time frame. until sources are no longer backwards compatible with hdmi 1.4.
I'm sorry to say, but for 'only' 500bux, you will never get leading edge technology in a tv.