I'd lean slightly toward the 250GTS, but honestly, I don't think you'll notice a heck of a lot of difference.
The 4800 series is known for generating a good amount of heat, but it's nothing to worry about if you don't have other issues such as a motherboard or case with an odd configuration that impedes airflow. It also gets progressively less troublesome as you go down the list; 4890s run hot, 4870s run a little less hot, and I've not heard of a lot of problems with 4850s.
If it makes you feel any better, I have a 4870 and a Q9950 chip in my main gaming machine, and there have been days where it's played Dragon Age and Crysis for several hours at a time with no heat issues whatsoever.
Also, what are the rest of your system specs? There are so many cards on the market that there may be better suited cards for you than the 4850 or 250GTS in any case.