I don't know if a bilge/switch combined unit is going be more reliable as I'd assume the difference would be negligible, but if either goes bad you have to replace the entire unit. With the separate setup, you can replace one or the other if they go bad. However, if I remember right, the switch is around $40 and the unit with both is about $60. You may be better off just going to the entire unit together, both for ease of installation (no separate wiring for the switch) and simple troubleshooting (if it's bad it's bad, take off and replace). Most new boats come with the combined unit, I don't think the separate switch is very common anymore.
I personally think a second bilge is overkill, and I have never been in a situation that needed one. However, I do have a pump for my extra ballast that I could use in a dire situation lol! If you were in a situation that required a second bilge to keep the boat floating from the amount of water coming in, I'm pretty sure you'd have some other serious issues and it would be time to abandon ship!