Two 'broken' iPhones, which way to fix?

Hi. I have two iphone 4. One works 'fine' but the home button does not respond - so we have to use assisstive touch. This iphone is from the day of launch - so has been around for a while! I was recently donated another iphone 4 - much fresher - however - I think this has had water damage (denied by previous owner) - it does turn on - but does not go past the connect to itunes logo even though I have tried DFU mode etc. Google tells me that either the battery has gone or the something power related. So my question is - which way to go! Which iPhone do I use for parts - or which do I fix? Do i take the home button out of one, and merely replace the home button on the older iPhone 4, or what would I take out of the older one to repair the possible water damaged iphone? Any suggestions? Thanks.