We have a connection of 1 hr 40 mn at IAH. Is that enough time to catch our next flight to LAX ?
Depends on whether the AMS-IAH flight is on time and the bags come out without delays.

Do we need to recheck our bags ? - or what is the correct procedure ?
Yes. You have to recheck your bags upon clearing customs in IAH

We would have an 2h 35m connection in Newark. Do you think this is a more reliable option ?
The AMS-EWR flight is shorter than AMS-IAH flight but you will even out on the EWR-LAX vs IAH-LAX flight. Personally I will take the connection via EWR as the 2:35 connection is safer.

Else since you are looking to book a single ticket, make sure what the next flight to your destination is irrespective of EWR or IAH connection.