The question is which uses less watts at idle.
Personally I'd go for the 5450
Get the X1950 board its a good amount faster. Im guessing AGP ? Can you tell us your rig we can help more... gl
The 1950 Pro would be better for games (I used to have a Sapphire 1950 Pro 512MB which was even powerful enough to play a demanding game like Supreme Commander) and the 5450 would be okay for anything non gaming.
If you are not a gamer then get the 5450. If you do game or want to game the X1950 Pro is better. I used to have a X1900 GT and it was about as fast as my 7900 GT and a little faster than my 7800 GT.
Power consumption is not a priority.
The HD5450 supports Directx 11. This seems to be its main advantage.
The X1950 Pro is probably faster for gaming. My son uses my computer for some gaming when is sometimes at home on weekend. Gaming performance is absolutely not a priority but would be an advantage if everything else is equal.