Bear
Really?
Execujet and Jetgo at least already successfully operate the types they are said to have bid and they would have a pretty good idea of the true operating costs.
Challenger vs Legacy both have been previously modified for SAR (or the EMB145 has), so not much risk for AMSA. Both have similar range, but the Embraer would be quite a deal cheaper to run and would carry nearly double the Challenger in the cabin which some ex AR guys I know reckoned was a stretch at times in the 328 with all the stuff they need to carry, so will be tight in the Challenger.