Horses for courses and all that.
The 850R would arguably be the "better" car. But not really enough to really make any difference. More of them about so lower prices too. For something to just run about in or play with that's probably the better option right now.
For collectors, the T5-R is a limited run special edition with the link to the BTCC campaign. The 850R is "just another Volvo". Yellow worth more than green worth more than black. Estate worth more than saloon. Genuine manual worth most of all.
Imports tend to be low miles, well kept, and rust free.
Where UK cars tend to have been to the moon and back over 25 years of salt encrusted winter roads.
Obviously generalising there but that's the gist of it.
As for "true" market values that's almost impossible to tell. But the rarer models will only get rarer. Even more so cars that don't have intergalactic miles, rust, or tuned and raced to death.
At the end of the day something is worth exactly what someone else actually paid. Regardless whether others think they should have got it for 20p
