My Quandry: English vs American English?

This is a problem that has been stumping me for sometime: whether I should be writing in English (as, being a Brit, I've learnt) or should I write in American English?

I know most people are going to say write in English, but here's my problem:

1.) Most of my readers are American. I know this because my sales are the highest on Barnes & Noble, a site only inhabitants of the US can purchase from.

2.) Many of my novels are set in the US, therefore my characters should technically be speaking in American.

Okay, the novels which are based in the UK is a no-brainer for me. They will be written in English. But should I write the US novels with American grammar?

This problem has been bugging me for quite some time. One reader even asked me if my main character in Alone had been born in England as her thoughts were very 'English'. Now, that one is a struggle. As much as I try to hear my character's voices with an American accent, not being American myself, there are bound to be things I don't quite get.

My editor now is American, so she goes a long way to picking up my mistakes: all of the 'towards', instead of 'toward'; 'earnt' instead of 'earned'; 'garbage' instead of 'rubbish'; 'sidewalk' instead of 'pavement'; and don't even get me started on the whole usage of 'pants' and 'fanny'! When someone is taking off their pants in the UK, or patting someone on the fanny, its a whole other thing!

So what are your thoughts, guys? Stick with English grammar, but American words when I'm writing novels set in American? Or should I go the whole way and turn everything to American?