Hard to say. Look here.
http://graphics.tomshardware.com/graphic/20031229/vga-charts-12.html
http://graphics.tomshardware.com/graphic/20031229/vga-charts-03.html#unreal_tournament_2003
http://graphics.tomshardware.com/graphic/20031229/vga-charts-08.html
I would say they are really close but the 9800SE seems a bit faster.
Im not sure if any games now use 256 mem where it makes a big
difference. That might be a factor but I dont think its that important
However here in this review of a Sapphire 9800SE they say its slower
than a 9600Pro
http://www.extremeoverclocking.com/reviews/video/Sapphire_9800SE_4.html
I think either way probably not a big difference between the two from
the charts. It depends on whether your particular card has some better
features like great memory that can be OCed more etc.
If its true they are fairly similar it depends on 128 megs vs 256.
When DOOM 3 was new many articles said few games used 256 and that you
only really used it with all the effects on and at high res.
Other aspects are overclocking potential. Some people do say that some
9800Ses can be unlocked - the pipes and overclocked then it might be a
better deal. Plus theres ALL in wonder aspect if thats what you want
the TV tiuner aspect if you want that feature.