Hi Tintin
The new standard 31.8 is quite new so we're not much concerned out here i think.
The pros of the 25.4 are
1) you already have it
2) it's lighter
3) you have more room to put your vetta computer and your vistalite lamps, bar ends and little bell
Cons
1) it's said to be less stiff
Pro's of the 31.8?
1) Stiffer if you need it (dh, enduro?)
2) a new satndard of the industry, pushing people to change the nice stem hb/combo they now have on their bike (certainly good for the industry)
con's
1) you guessed it less room on the handlebar (i think it could really get problematic on a straight bar with twist shifters and barends)
2) heavier
I think this standard is here to stay though, i just hope they will continue to produce 25.4 for cross country.
Bottom line? if your new bike comes with it, don't change it; If you don't have it on your "old" bike : don't be sad.