Big hiccup...none of the tile substrates are rated for application directly over dimmensional lumber - you need at least a 1/2" of ply, C-faces or better (i.e., no D side) installed across the joists, and exposure I or exterior glue holding the ply together. Then, lots of screws to hold it down (screw any loose planks down before you start). Then, and only then, IF the joists are sufficient, can you tile things. You can use the Wedi, but are you sure it is rated for having heat applied over it? The amount of insulation 1/4" would provide is marginal, and probably not worth the extra cost. You have a bunch of choices on decoupling, you could install cbu-mat-tile, or tack the mats to the ply, put slc over it, then a decoupling membrane like Ditra, and then the tile. You can also check out www.johnbridge.com for some ideas on tiling. SLC over ply generally needs to be at least 1/2" thick over the highest point. If you install cbu first, you can put a thinner layer of slc if you wish to go that way. It's MUCH easier for a first-timer to get a good slc pour if it is thicker rather than thinner. Just like your pancake batter doesn't flow to fill the pan keeping a certain thickness, slc does too. Only if you get enough down, break the surface tension, does it actually flow, and then, only if it is thick enough. But, for a novice, getting a nice flat floor by trying to spread thinset out over mats without nicking wires is not the easiest thing, either.