A rule of thumb
When a parent has a reference to a child object, you should use a strong reference. When a child has a reference to its parent object, you should use a weak reference or a unsafe_unretained one (if the former is not available). A typical scenario is when you deal with delegates. For example, a UITableViewDelegate doesn't retain a controller class that contains a table view.
Here a simple schema to present the main concepts.
Suppose first A,B and C are strong references. In particular, C has a strong ref to its parent. When obj1 is released (somewhere), the A reference doesn't exist anymore but you have a leak since there is a cycle between obj1 and obj2. Speaking in terms of retain counts (only for explain purposes), obj1 has a retain count of 2 (obj2 has a strong reference to it), while obj2 has a retain count of 1. If obj1 is released, its retain count is now 1 and its dealloc method is not called. obj1 and obj2 still remain in memory but no one has a reference to them: Leak.
On the contary, if only A and B are strong refs and C is qualified as weak all is ok. You have no leaks. In fact, when obj1 is released, it also releases obj2. Speaking in terms of retain counts, obj1 has a retain count of 1, obj2 has a retain count of 1. If obj1 is released, its retain count is now 0 and its dealloc method is called. obj1 and obj2 are removed from memory.
A simple suggestion: Start to think in terms of object graph when you deal with ARC.
About your first question, both solutions are valid when you deal with XIBs. In general weak references are used when you deal with memory cycles.
Concerning XIBs files, if you use strong you need to set nil in viewDidUnload since if you don't do it, in memory low conditions, you could cause unexpected leaks. You don't release them in dealloc because ARC will do it for you.
weak instead doesn't need that treatment since, when the target object is destroyed, those values are set as nil automatically. No dangling pointers anymore.
If you are interested in, I really suggest you to read friday-qa-2012-04-13-nib-memory-management by Mike Ash.
About your second question, if you need to support iOS 4, instead of weak you have to use unsafe_unretained.
Within SO there are a lot of questions/answers. Here the main ones:
How do I replace weak references when using ARC and targeting iOS 4.0?
What kind of leaks does automatic reference counting in Objective-C not prevent or minimize?
using ARC, lifetime qualifier assign and unsafe_unretained
strong / weak / retain / unsafe_unretained / assign
Hope that helps.
Update
As per shaunlim's comment, starting from iOS 6 viewDidUnload method is deprecated. Here I really suggest to see Rob's answer: iOS 6 - viewDidUnload migrate to didReceiveMemoryWarning?.
