158's are just great everyday units, do everything they are supposed to do & more. Most as said are suffering due to the way the TOC's use rolling stock these days. i.e. run everthing into the ground & replace with cheap stuff more often than actually looking after what they have already. (same thing happens with 66's on the freight side).
Our 158's are just getting enough attention to keep going, some of them are getting very tatty inside. They kind of got ignored a bit when we got 175's as they were thought to be the next best thing, now they have proven how awful they are it seems our 158's are getting a bit more attention again. The ex central & scotrail ones we had a few years ago were absolutly shocking. They had been well & truly run into the ground. Thankfully we only had them whilst the ongoing 175 problem's were patched over.
170's well there a tad too bland for my liking, kind of like a mondeo car. Get's you from A to B, but isnt' special or really that nice. Also the seating/door layout favours chavs & other d*ckheads who can basically take over a whole section & ruin it for everyone else.
When they have been used & abused for as long as the 158's have then I think you will be able to compare the two better. I will also predict that the 170's either won't last as long as the 158's have or will be a lot worse off than them. Like our 175's, the LM 170's around my way are all falling apart inside already. The new build of 170's looks to be even worse due to the new bogies they will be using.