I think you should find a size that allows for your current needs plus a lot of headroom -- and then get MULTIPLE drives to back up. I use 4 drives for maximum data redundancy (I lost everything 3 years ago).
1. System Drive (including Music, Videos, Applications, etc.)
2. Aperture Library
3. Aperture Vault
4. Time Machine (for Drive #1)
I don't want Aperture and Time Machine talking to each other for fear of corrupting one another. This solution makes certain that if any drive goes down, I don't lose any data.
One could use 2 or 3 drives instead of 4 (if your Aperture libraries, music, etc. aren't too big) but I prefer to have everything separate. Internal SATA drives are so cheap now that it is silly not to have multiple drives protecting your data. Just imagine if you lost everything