The WiSM fulfils the same role for the LWAP systems that the WLSM provides for the stand-alone systems (and offers a larger LWAP count than a pizza-box controller) ... they each have their place ... I don't think Cisco will discontinue the WLSM anytime soon.
As for one system versus the other ... I believe we'll see both systems (LWAP and standalone) for quite a while.
As a "worse case" (bad choice of words, sorry), I think we'll see the stand-alone systems continue for while.
First, for the users that only need a small system, the stand-alone system is much more cost effective ... a couple APs, light one up as a WDS, and the system is good.
Also, because of their automomy, there is no single point of failure (of a properly designed system where redundancy has been planned and implemented). I haven't see much printed about what happens with the LWAP system when the controller dies ... especially when it's operating in L3 Mode.
I'd have the same concern for the stand-alone system using strictly POE ... the switch dies, so do all of the APs ... it's their power source. It might be a good design to have some critical or redundant APs fed by a power brick locally instead of using only POE.
Reasonable cases can be made for either system .... so, historically, Cisco will provide both to better cover the market.
There's also no reason (other than costs) that would limit you to one system or the other. In a critical environment, perhaps it would be prudent to use one system as a redundant system to the other and cover all of the bases.
Airespace is a great system, so the the Aironet ... I'm betting they'll both be around for a while, and choosing either would not be considered a career limiting move.
Just my .02
Good Luck
Scott
