Assuming the concrete flloor is clean and not moving (active cracks), there's no reason to use an underlayment. Schluter makes great products but none of them are better than a clean, flat concrete slab. If the floor is actively cracking, I would use the Schluter as it helps float the tile floor so any cracking beneath it won't transfer to the finished grouted tile.
Again, assuming the floor is flat, clean and stable.. I'd use a polymer modified thinset directly on the concrete. Unmodified will work but modified has better bonding properties and is easier to trowel and clean up. IMHO, it also sets a little slower allowing for more working time. I'm not a pro so working time is important to me. A pro can mix a bucket of thinset and knock it out before it sets up... but a pro also has someone cutting tiles, cleaning tools and buckets. I try to only mix enough thinset that I think I'd use in about 40 minutes.
Some tips:
* If you are using a light colored grout, use white thinset. If you are using a dark grout, use a gray thinset.
* I've done quite a bit of tiling and have learned that I use a wet-saw for about 1% of my cuts. 90% are done with a simple snap cutter. Lowes has a decent one for
$119.00
*Most small notches or circle cuts can easily be done with a 4 or 4-1/2"
Diamond Blade on a cheap angle grinder. I think I pair $19.00 for my black and decker angle grinder and have been using it for years.
* I do own a wet-saw but rarely use it unless I'm working with marble or I need to make a cut which is either too risky or impossible to do with a snap cutter or diamond blade on the angle grinder.
* For floor tiles (particularly large format tiles), I would only use porcelain tile. It is much less prone to chipping and cracking vs. ceramic tile.
* I've become a big fan of tile lashes for large format tile. It will make a novice's job look like a pro did it.
Tile Lash part A
Part B
You'll still need to use your desired sized spacers with tile lashes.
