How "difficult" the practice is will depend entirely on the firm's clients, the type of work they bring in, and how much work there is. I don't know that there's a stereotypical hardest or easiest.
As for pay, this will probably vary widely depending on the firm. Everyone is going to get the same base pay if you're the same year. However, you're more likely to make and/or exceed your minimum billable requirement if you're in a busy practice group, leading to a larger bonus (assuming that the firm's bonus structure gives varying bonuses for how many hours, instead of everyone who meets the minimum getting the same bonus and that's it)
As for job security, I think the same would apply. If you're in a busy practice group, the work will be there for you to do. If it's slow, and/or your work is crappy, they may let you go (although it can be hard to get fired, from what I hear, and firms usually only do it if things get really bad economically (which happened after the dot.com bust) or if you do really, really crappy work.