An "Archive & Install" or a Leopard "update" will preserve the apps already installed and just add/replace the necessary bits.
An "Erase and Install" deletes everything and restarts with a clean slate. Since the Leopard OS doesn't include the "bundled apps" that came with the original install disks, they'll disappear -- but they can be re-installed later from the Tiger install DVDs.
IMO, clean install is the way to go -- because it gives you an opportunity to get rid of many GBs of useless "bloatware" and applications that you'll never use. For example, the iWork and MS Office 'Test Drive' packages consume 2.5 GB -- that's more than all other "bundled apps" combined. Also, I currently have no use for iWeb or GarageBand, so I'll save several more GB of HD space by leaving them out until I need them (if ever).
I just did a custom clean-install of 10.4.8 -- with ONLY the required "core OS" components and absolutely nothing else. Total disk space = 2.87 GB. Next, I grabbed all the recommended Software Update packages and moved up to Tiger 10.4.10. Total disk space = 3.11 GB (including "core" apps like Safari and iTunes). IIRC, the original factory pre-load consumed nearly 20 GB.
...don't install it 'til you need it,
LK