Which Passport?

Sorry, this has probably been asked before but I couldn't come up with anything in a search.    My 9 month old has both a UK and a US passport.  We'll be flying to the US in a few weeks for holiday - which passport should I use?

From what I understand, I need to use his US passport to get him into the US so can I assume that I should use his US passport to come back into the UK?  Do I need to bring his UK passport?  I'm confused

You use his US passport to enter the US, and his UK passport to enter the UK.  He is a UK citizen, so he needs to enter the UK on his UK passport, not his US one.  If you tried using the US one, they will likely ask you where his visa/entry clearance is.

Thanks Geeta.  Should I not worry about the stamps then?  I was concerned that he will get stamped when we enter the US but as he won't get a stamp in his UK passport, will it not look like he never left the US?  Does that even matter since he's a dual citizen?

The stamps won't matter. In fact, I don't think US citizens ever get stamped when they leave the US...do they?

No.  I never have. 

My daughter (dual UK/US with both passports) has been back to the US twice in her young life (22 months) and has never been stamped in/out by either the UK or the US. It was only on this past visit that I asked the IO in the US to stamp my passport (so I can prove when leaving the UK & returning for my citizenship application) that I asked him to stamp my daughters US passport...and he did, he said it's not part of their procedure to stamp in US Citizens, but they will do it if you ask them to.

That must depend on the airport/IO in the US because I have always been stamped on my return to the US and have never asked. 

Maybe they see my UK visa and figure I need the stamps?

I've always been stamped in but never out.

That must depend on the airport/IO in the US because I have always been stamped on my return to the US and have never asked. 

Maybe they see my UK visa and figure I need the stamps?

I have only flown the LHR - SFO direct route, so it could vary from each point of entry because each time I've entered the IO asked if I was a resident in the UK and I've shown my visa and the response was 'oh ok', no stamp unless I asked for it.

Okay, thanks very much.