I think it's quite close.
158s work the Cambrian Coast, Wick and Kyle lines, the North Wales Coast, the Settle and Carlisle, Leeds to Norwich, Nottingham to Skegness, Sheffield to Bridlington, Liverpool to Norwich, Edinburgh to Aberdeen/Inverness, the Borders line, a few jaunts in Devon, the Calder Valley line, Cardiff to Brighton, Malvern to Weymouth, and the Scottish central belt. They have also done the WCML and Birmingham-Swindon for Virgin; they've been on numerous Alphaline jaunts to Waterloo; they once operated Rochdale-Euston services for FNW and used to run on the TransPennine Express line. And they also run in Thailand, though I doubt that counts.
The 150s, however, have been almost everywhere; the Settle and Carlisle, the HoWL, the Cumbrian Coast, pretty much the entire West Country, large parts of the West Midlands, a lot of Lancashire and quite a few lines in Yorkshire. They don't push as far into Scotland, however.
The 156s have operated in Scotland and large parts of Northern England, but not much else. The 153s are mainly limited to the North, the Valleys the Midlands (East and West) and East Anglia; again these units don't push into Scotland.
I would say that the 158s serve the most stations today, but the 150s have served more different stations than the 158s since the latter's introduction in 1989.