Hi, for what you intend to use the drive for, I would go for the Western Digital 500Gb.
Yes you could use it to back up two computers using Acronis True Image. Each backup will be easily identifiable via the Back-up icons.
When making your back-ups, I would advise you to make an initial Full Back-up and then thereafter make Differential back-ups on say, a weekly basis - you can schedule the automatic differential back-ups. This is how I make my back-ups, I then delete earlier Differentials, just keeping the Main Initial back-up and one Differential - this saves space on the HDD.
I would advise against making Incremental back-ups, because in the event of actually needing to use your back-ups you will need to have your Main Back-up and ALL your Incremental back-ups (like a daisy-chain), any single Incremental getting corrupted would render the whole lot useless.
You do not need to set-up the HDD in any particular way, just format it NTFS. Each backup will create its' own icon on the drive.