Mixed feelings, really. I guess my preference more or less depends on the number of slots one plans to have available at the start of an RP. If there are good number of slots, like say... eight or more, I kinda see where a reservation is more warranted. If it's a smaller RP, like six or so, then I kinda think the SUs are better in a more "just post a sign-up and there will be judging a few days later" sort of thinking.
^ Basically that! Sandbox-y, open roleplays can have reservations, or open, sign-ups whereas smaller, more intimate roleplays should have some kind of selection or more rigorous SU phase.