Much better to install the official debian ones - they are less likely to mess with your system, and whoever is releasing them seems to keep up with the releases, so you are not behind.
Less chance of interfering with an overlapping package supplied by the distribution.
At least, /opt is part of the FHS, to which Debian adheres.
If it uses the package manager it goes in /usr like a regular program.
Eg, say Debian include Chrome (as Chrome, not Chromium) in their next release. It will move from where it is now to the normal locations for unimportant software beneath /usr.
That said, I don't really care that much anymore. There's very little expectation for quality finished software on Linux in general:
* Most apps aren't packaged, even for the two major distros people use
* half of GUI apps still don't install with shortcuts
* WMs still invent their own .desktop replacements
* package descriptions for 'foobar' are 'the foobar package' or 'libtoolkit GUI for libfoobar'
* service descriptions are 'starts the foobar service'
* man pages either don't exist, point elsewhere or consist of 'debian says we have to have a man page, this is a man page, alas it has no content'.
In addition, I'm really happy to see Apple and Google keep cooperating on making WebKit better. Together you've now built the best desktop and mobile browser technology.