I just ran a little experiment:
1) SPM: 21634 packages listed, 1134 installed
2) Update Manager: 108.6 MB updates available to download
3) After sudo apt-get update:
Update Manager: 246.8 MB updates available to download
SPM: 21634 packages listed, 1134 installed
SPM Reload: 21643 packages listed, 1134 installed

The significant thing that was updated was the package list available
for download to Update Manager, though it appears that after the
Reload on SPM there was no difference.

It appears that SPM that I have installed is the latest release
available 0.57.11.1ubuntu14 which may account for why SPM does not get
upgraded whichever method of upgrade is used - i.e. because it does
not yet have an upgrade at its current level of release (for Fiesty,
and is probably not going to get one with Gutsy Gibbon just around the
corner next week).

Since I never run Update Manager because there is no fine grain
control unless you uncheck everything which looks daunting, I most
often choose to use SPM.

Yes, aptitude handles the dependencies and as you say unless you
install with it you should not try to uninstall with it.

I never implied that: sudo apt-get update would do any upgrading at
all, only update the available repo lists you computer knows about
before attempting to do an update. No upgrades implied by my post.