Hi Unreg:
Great question. The ultimate answer is still house specific as some venues consider the straddle to be a new BB (for preflop action only) while others consider it to be a raise on top of the BB which allows the straddler to have last action pre-flop.
Assume in both cases the allowed straddle is 2X the BB, so at 5 / 10 blinds the straddle is to 20 in both cases. In the first case the min raise above the straddle is 20 for a total of 40, and in the second case it's 10 for a total of 30.
One key to your question was discussed at some length at the 2013 Summit: that declaring "5" in your example is exactly equivalent to pushing out a mixed chip bet totaling 5. Some people would construe a difference, that is that declaring any amount over the amount to call is an indication of intent to raise; however that would leave us with a situation where pushing out five 1's might be treated differently than declaring "5". With that in mind, take a look at the two cases using the values in your post:
In the first case, where the straddle is considered the "new BB preflop", pushing out a total of 5 in chips would be a call, because the min raise is 4 (for total of 8 ) and half a min-raise (the min threshold for a chip push to be considered a raise) is 6 total. The guy just said / pushed 5 so that's less than 6, so it's a call per TDA Rules 41-A and 43.
In the second case, where the straddle is considered a raise over the BB, the min raise is 2 (for total of 6), and half a min-raise would be 5 total. In that case, the guy says / pushes 5 he has min-raised and must make it 6 total.