Right now slim and bootcut are primarily what I'm seeing on the street ... the trendiest at the moment are the super skinnies. But, straight or bootcut jeans are the most flattering in general ... almost anyone can wear them and look good. Then again, flares and wider cut jeans are very flattering to the wide hipped girls who need an illusion that the hips are only sightly wider than their legs. I think that various widths will probably always be around just for that reason. Better to have jeans that flatter your particular shape, than to look "trendy" but weird.
However ... there's every indicator (lot's of designers have show them for this spirng) that it's going to change to flares. Not the radical flares of the 70's ... slimmer in the leg with just a sight flare at the bottom. Not sure the average person is ready for this yet, or not ... probably not.
Jeans styles change about every 5 to 7 years .... remember several (5 to 7) years ago when every one was wearing super low rise jeans? I said then, that we'd see higher waisted jeans soon (not the high waisted "Mom jeans" of the 80's, but definitely higher than the top of your pubic bone where everyone was wearing them, then) ... and everyone said they'd never wear them. But guess, what ... they are wearing them now. And now, baring the tummy like that seems so ..... yuck!
Of course, I find that it usually takes a few years for the general public to shift their attitude about a change in shape from the time designers start to show them. So ... In spite of them saying that flares are hot this spring, I don't think the average person will make the move at least until next year ... but it's probably even a couple years away.
What will swing the early adopters is seeing them on celebs.