OK guys I could go on for pages and pages on why the STi pans are better, but I'll try to keep the rambling to a minimum.
Here are the 3 pans. OK, 4 pans, had to give myself a shameless plug
Obviously the WRX pan is flat bottomed and not as deep, vs the STi pans that form a closer fitting 'tub around the pickup. This gives the oil less pleces to slosh around to, and keeps the oil level higher on the pickup during operation.
Top Left: Killer B Motorsport Oil Pan
Top Right: OEM 2002-2005 EJ20 WRX Oil Pan Subaru number 11109AA053
Bottom Right: OEM 06+ EJ25 Impreza WRX/STi (STi, Legacy GT & Forester XT) Subaru number 11109AA151
Bottom Left: OEM 04-05 EJ25 Impreza STi Subaru number 11109AA131
As most of us know, the STi pan can be used mith many more header setups do to the kicked-in feature.
The dipstick location and angle, shape, length etc. is the same on all pans. This is the little tube ON the pan. The only difference is the length of the tube that the dipstick sits in. It's different on the EJ20 vs EJ25. That's why if you just change the dipstick when swapping to an STi pan your readings are off. I'm not entirely certain if the brackets on the dipstick tubes are the same (I'd think they would be), but swapping dipstick AND pan should give proper readings.
On to the baffling... You can see the WRX baffles have fewer passages and 2 levels of baffles. The STi pan has a much more clear flow path. Keep in mind one of the achilies to the Suby lubrication system isn't oil control IN the pan, it's GETTIN the oil to the pan quickly as it flows back from the heads. It doesn't do you much good to have good oil control in the bottom of the pan if the oil isn't getting there.
A lot of the perimiter of the WRX top baffle is sealed to the sides. On the STi pan almost the whole perimiter of the top baffle has a flow through gap. Also tough to see in this picture is that the lip around the baffles (upper and lower) pickup hole is curled up. This forces the oil to one of the hols in the baffle instead of going right down the middle.
Here's another shot of the EJ20 double level baffle. It's tough to tell by this picture but it's mostly sealed around the edges. The flow through path takes many direction. I'd be willing to bet, because most of the sides are sealed on the lower level, that under hard accel, decel, & corners that you can pool oil in some locations between the 2 baffles.
In case anyone cares, the weights go something like this: EJ20-heaviest, 04-05 EJ25 next, 06+ EJ25 a little lighter and the killer B Oil pan a little lighter than the 06+. This picture shows the 04-05 & 06+ EJ25 pans. IMO the 06+ is a better choice, because it leaves half as much oil in the pan when you do an oil change, it's lighter, and the baffling is slightly better (although I'm probably splitting hairs here). Just an FYI on the 04-05 pan there is 1/4 quart that doesn't come out during an oil change and a baffle around the inside of the drain plug so you can't pump it out.
Something else to note is that the baffle tray (sheetmetal piece that bolts to the bottom fo the block) should also be changed when doing an oil pan swap. They are cheap, and specifically designed to work with each pan.
Hope this helps