Difference between osm https://github.com/openstreetmap/mod_tile.git and https://github.com/SomeoneElseOSM/mod_tile

I made the change a couple of switch2osm releases ago. It made no sense to have everyone following the switch2osm "manual" instructions (which are designed to be as simple as possible to follow) all making the same changes to config files. There are 3 active branches of https://github.com/SomeoneElseOSM/mod_tile - master, which tracks the parent repository, switch2osm, which incorporates some minor changes to config files to match the switch2osm instructions, and zoom which incorporates some further changes to support zoom levels up to 24. See this diary entry for details. The paths used in the metatile cache are different in the "zoom" and other branches, so it wouldn't be possible to make that a "switch2osm standard" without everyone deleting their tile caches.

As to whether you'd want to follow switch2osm or Ircama's instructions, it's up to you. The switch2osm guides deliberately use packages distributed as part of the OS if possible; Ircama's guide suggests using later versions from elsewhere in some cases. For example, if you want to use Mapnik 3.0.19 on Ubuntu 16.04 Ircama's guide explains how but switch2osm doesn't, in the interests of keeping it simple for people who have not set up a tile server before. If you're installing on Ubuntu 18.04 I think you'll actually end up in pretty much the same place whichever set of instructions you follow. One possible diffeence is that the tile expiry scripts in the switch2osm version work out of the box; the parent ones may not.