The 350 uses the older 15 version of the MPEG2 encoder. This was much hotter - to a point that some people claim their boards had hardware failure after some time. Older 250's and I believe all 350's use this. Newer 250's use the 16 verison of the encoder. It runs much cooler.
Keep in mind that if you have a 250 and a 350 with different encoders you may have some driver issues installing both in the same machine. SnapStream does not support multi-tuner nor does the bundled Hauppauge WinTV so you would not be able to test with that method anyway.
BTW, I run with three of the older 250's in my machine and while it does run hot it has not failed yet
