The only cases of parylytic polio in the UK for a number of years (can look this up but think its at least a decade- possibly considerably longer) have been contracted from the vaccine itself.
The US which uses the killed version says that it is perfectly effective. The UK says it isn't. I susepct it is less effective than the live but not sure how much by.
If you went to places like the US (and I think a lot of S.America) you wouldn't have to worry about polio- they're in the same position as us. Personally I would give ds2 polio if we were going to India (very unlikely considering we have ds1!) but otherwise I'm not worried about him not having had it (and some of them I do worry a bit about him not having had).
My owm view is that for the child the live virus is better as it takes the normal route into the body- so a normal immune response is produced. ie it is normal for polio to enter through the gut.
Basically I agree with Tamum- it's the apporach we decided on for ds2.