Hi rustygirl,
I would stick to one test, first response is all I ever use and I've NEVER had a false reading from it. I got my BFP this time at 8dpo, and last time at 9dpo.
I'm not sure about the differing results? Did you use the same urine for both of them? I know First response is really sensitive.
I would test again with first response tomorrow morning ( and this arvo if it was me ) I have always gotten my BFP's in the arvo/night so just hold on for a few hours and don't drink alot so the urine isn't too diluted.
Good luck (and a quiet congrats from me xxxx)