It is a pesonal choice -as for what to say depends on how you feel. To people who are just going to give me their opinion/ideas rather than have a discussion I usually say - Thanks, that is interesting/a good idea.....insert other positive but non committal response, with no intention of following through. It may seem like giving in but honestly sometimes these people just want you to acknowledge them and their experience. If someone is interested in how things work and has some hints/ideas rather than judgements then I would perhaps flesh things out further - generally though these are people who share the same ideas or who I know will be supportive no matter what.
DD1 is a shocking sleeper - still at 3. We never really coslept. DD2 has coslept pretty much since birth and she has just transitioned to a cot recently - as it would appear we were keeping her awake, lol. She is a fabulous sleeper and a really settled baby - but that could just be her personality!
All bubs are different - all families have different needs and ideas. You should do what suits you both best - honestly I wish I had ignored all the 'helpful' comments with DD1 and trusted my instincts. You are doing a great job!!!