For the back, if you have a RC slab, you can plan for future extension of your back rooms, etc. For the front porch, having a RC slab in place of the roof tiles look nicer (in my own personal opinion)
Of course, the RC slab will be slightly more expensive that the timber roof with tiles. Just make sure you put in water proofing for the RC slabs. Depending on the size, the additional cost will not be too much.

If going for exposed roof truss then use good quality timber with varnish. This will bring out the wood grains. Since using exposed roof truss, might as well go for the clay roof tiles. The bottom of the roof tiles are the same as the surface. With these, it may cost more than the RC slab
All down to individual preference and the house theme

OK. If want cheap, then have to compromise..
use treated timber and the normal concrete roof tiles.
Paint the back of the concrete roof tiles. This want a friend of mine did.

Hahaha good suggestion