The only difference should be that the bigger one includes the /source directory of the
Slackware File Tree for your version. Use either one, both contain the same installer. However, better check that the one you use comes from
here to make sure it's a genuine one.
And preferably use the most recent stable version that is 14.1, 32-bit or 64-bit.
Bear in mind that Slackware is a binary distribution (however the /source directory gathers all that's needed to rebuild the packages), stable and not bleeding edge. To grasp the differences with Arch you could read
Get acquainted with Slackware. More generally,
SlackDocs intends to be the primary source of documentation about Slackware Linux.
To write the ISO on an USB stick, just use dd as the ISO images for the version 14.1 are already hybrid.
Don't be afraid, using Slackware should be very easy considering your background and welcome to SlackLand.