I'd choose the 850R over a 740 turbo. The P80 series has more creature comforts and is easier to extract power from. I also think it's getting very hard to find examples of either model that are worth buying.
Speaking from owning a hooptie '88 740 and a decent '00 V70R, they are hard to find in decent shape. But for what they go for, you have lots of room to clean them up. I paid $300 each for those two cars (and my '98 too).
I can't say much for the 740, I've only owned mine for about 2 months and I've never driven it, but I've had my V70 GLT for almost 3 years, and I love it. It's very comfortable for road trips and it handles reasonably well for what it is with stock suspension and wheels/tires. The cars have lots of potential for power and room for handling upgrades. Just search around on Volvospeed, you'll find some crazy stuff.
I agree with the brakes being less than adequate for spirited driving, but I also have cheapo Centric 280mm rotors and pads. They work, but not too well.
As for interior, yes, more creature comforts. The newer P80 interiors are nicer than the older 850 interior, but they all have dash mount problems. My car's dash is incredibly noisy and irritating. My 740's dash hasn't fared too well either, the glovebox totally fell out.
Maintenance cost on pre-1999 P80s is not bad at all. No ETM to worry about, no CVVT, and less finicky electronics. My cousin has a 240k mile '99 S70 T5 and it has its moments, but overall it's reliable. That car was just a victim of poor maintenance and abuse.
But asking this question on here and on VS you're going to get totally different answers. TB is primarily for red block/RWD cars, VS is primarily for white block/FWD cars.