I've had both, ST24 and ST200, and think the 200 is by far the superior car. It has 35bhp (+20%) and 15nm (+7/8%) more.
But there's more to it than that, it's how it feels to drive and how puts it's power on the road that makes a difference. It just feels faster, more focussed, you really have to work the 24 to get the power out of it, but the 200 always feels up for it.
I would agree that the ride is a lot harsher in the 200 though. Stiffer suspension, bigger wheels, thinner tyres do that, but it does handle the better for it.
However the best way to decide is what to you want.... more sporty or more comfort. The 24 is a good compromise of the two.
When I was looking for a car a couple of years ago I was looking to go back to a ST24, only bought the ST200 as I couldn't believe I found one in my budget.
But glad I bought it now. It's a great car if you can live with that ride, but the 24 is a good car too.
Good luck with your search.