I spent a good few years thinking straight leg meant boring leg, the jeans equivalent of oatmeal, and I stuck to a bootcut because it felt like it did more for me. What put me right was a pair that actually sat properly on the hip instead of gripping it, and I realised the shape wasn't the problem, the cut I'd been buying was.
A good straight leg pair works with almost anything you'd normally throw a bootcut or skinny at, but it does it with less fuss. A jumper tucked in sits better because there's no flare to balance out. A boot works, a loafer works, even a plain trainer works because the leg doesn't taper in and fight with the shoe the way a skinny jean does. I wear mine with a long cardigan and nothing else trying too hard, and that's most days sorted. The cheap ones let you down here because the leg is either too narrow through the knee, so it's not straight at all, it's just a softer skinny, or it's cut so wide it reads as a different trouser entirely. A proper straight leg keeps the same width from thigh to ankle, give or take, and that's the whole point of the thing.
Fabric is where you feel the price difference fastest. A thin stretch denim with no weight to it will bag at the knee by lunchtime and never recover, whereas a denim with a bit of substance, even with stretch in it, holds its shape through the day and across a few wears before it needs a wash. Look at the inside seams too, a cheap pair has them pressed flat and fraying within a season, a better made pair has proper flat felled seams that don't curl. The hardware matters more than you'd expect from something nobody sees, a button that's just stamped metal will eventually seize up or the thread around it will go, a riveted one lasts years. For fit, straight leg forgives a lot through the hip and thigh but it's unforgiving at the waist, so try sitting down in them before you decide, a gap at the back waistband when you sit is the fit problem that ruins an otherwise good pair. Go a size down if you're between sizes rather than up, because they stretch with wear but they never shrink back on their own.
The ones that start a bit stiff are usually the ones worth having, because they soften properly with washing, the colour fading unevenly in a way that looks earned rather than just worn out, and the hem starts to fray a little at the heel in a way I've come to like rather than mind.