Dave,
I have used, and have seen, Elite 4200s used on a variety of calibers, from light .243s to .375H&H Magnums - They have ALL worked well. The biggest gun I have seen used with a Conquest was a .300 Win. Mag., and it too performed very well without any problems.
I have not heard anything about the Conquest having internal components that were either weak or less than what most shooters would like. Although I really like the Elites, I honestly can't say any real negative things about the Conquests.
HOWEVER, please understand that the Conquests are still relatively new models. As such, it will be a while before they have gone through the test of time - like Leupold Vari-X IIIs, etc.
You may say the same thing about the Elite 4200s, but not really. Specifically, although they are under the Bushnell name, they were first under the Bausch & Lomb name for a much longer time. They used to be named the 4000 and 3000 series.
Which one should you get? Get whichever one you want. But let me put it to you this way - I own more Nikon Monarchs and Bushnell Elites than any other brand, and my favorite is the Elite 4200s.
Zachary