Several factors you would need to consider:
1) Availability - how good / bad to find seats.
On that note, only UA's partner seats can be searched online. Both AA and DL you have to use the work-around. AA - use the BA site. DL - use the AF site for seats on AF/KLM.
To successfully redeem DL saver award, it is a MUST to learn about the How To by reading / following the STICKY in DL forum. There are some amazing awards people unearthed, despite the SkyPeso name DL miles have gained.
2) Change fee. AA is the most generous. UA is reasonable. US charges $150 for anything regardless.