Mountain bike derailleurs have a lot stronger springs than road bikes... I've also seen a lot of mountain bikes with a bash guard for the big chain ring, so they're not really triples. I don't go into my big chain ring usually mountain biking, just stay in the middle or the granny.

For avoiding sora - I'm more saying avoid 8 speed with a double, I've never tried sora or tiagra. I can't see not wanting to upgrade that soon.

I haven't noticed any difference in the shifting abilities of my bikes that are ultegra or full 105, also haven't had any difficulties with the front derailleur needing trimmed regularly on the 105. I have one bike that's a 105 level triple & 2 bikes that are ultegra doubles.