You'll stay on that airplane for 4 years, probably. They let people move in the past, but that was to fill a new fleet, so unless globals suddenly show up or something you'll stay in the same fleet.
The daily pay is the same, but the challengers do go international every now and then, so thats $100 more per day. But dont bet on that happening a lot, I think most people average 4 international trips a year. Plus, in the challenger fleet you'll work more overtime cause that fleet works its butt off. The 60 and 45 folks fly a lot less, have shorter days, thus less overtime. The 300 is a nice plane, but you'll work a lot more, and no matter what you'll upgrade to the 45 or the 60, so starting as an FO in the 45 or 60 has its benefits too. there really is no "bad fleet" to get stuck in.