The obvious answer here, and I'm surprised no one has posted it yet, is get both.
The DM42 is available now and a wonderful machine, with pretty much universal praise. Even if you've never owned an HP-42S, if you know/like RPN, you will find it easy to learn and use, and in time with practice, even the advanced features (e.g. matrix, solver, complex numbers) are pretty easy to learn and use.
The WP43S will contain a similar/familiar but distinctly different keystroke programming language, and similar precision (as the DM42S) but also more features such as additional statistic functions, likely more/better solvers, additional Math functions (many of which I don't understand), roughly similar PC data exchange capabilities, and more. If I were a marketing guy, I might call it a WP34S on steroids, but I'm not, so I won't, but that should complete the idea.
Best GUESS is the WP43S will not be available for 2-3 years, unless some talented developers appear with LOTS of free time very soon, but it will certainly be worth the long wait.
So, yeah, pick the best choice and get both; very unlikely you'll regret it.
DM42: β00071 & 00282