Here is an interesting thought to pounder: (to me anyways)
EP rules state a cars minimum weight is determined by displacement x 1.05. No car shall be under 1600lbs. The 1.6 (Si motor) cars must weigh 1680. The 1.5 (DX motor) cars can weigh 1575 (but need 25 lbs for minimum weight, which can be placed anywhere). Which would you do? Thats 80 pounds. At what point does HP have the advantage? Autocrossing anyways.

And of course money is speed...

 

I'd take the D-series motor....

The B16's are pretty gutless till 5000 rpm.

 

Ahhh, by Si motor I meant the D16...

 

The answer is it depends.

How much weight have you gotten out of the car? My 3G 86 Si, minus ballast, gas and the rollbar is around 1590. The windshield is still OEM glass, but the door window are out, and the hatch and rears are lexan. To get it any lighter would take serious effort. Yours is a 5th gen? You got more metal than me; I'd say it would be tough to get below mid 1600's, so you might as well stay with the D16, and if weight is needed add it in advantageous locations.

 

I'd go with the lighter 1.5 DX motor and place the 25lbs of ballast to the lowest part of the car. And you can probably play around to find out which location works the best accroding to course design.