OK, I see two different issues here, and the guardianship itself is only one of them.

The other issue is that there is a big difference of opinion here, and you and your partner don't have an established method of resolving it. It's not ok (in my opinion) that he went ahead and talked to his family without reaching agreement with you first. BUT he may think that because he told you he thought his parents were better that meant you'd discussed it (that's what happens in our house). Maybe talking about the way you talk, agree, and disagree will help clear up whether there was a misunderstanding or whether he has just ignored your opinion. In any case, you need to discuss how you and he are going to proceed in situations where you both disagree strongly.

Re the guardianship - I think you should sit down and list fully what the pro's and con's for each set of parents are - and include all the little details. There may be some reasons why each of you don't want the kids to go to the other parents - but you're being too diplomatic to say it. In this case you need to be honest. Things like "your parent's house smells' or 'I trust your mum/dad but I'm not sure about the new wife/husband' are all valid and need to be taken into consideration. But at the end of the day, what is most important is that there is someone who will love your children unconditionally.