98 and 99 both have the MAF sensors, but the one in the 98 is stonger (hot wire, as opposed to a hot film in the 99), and is much more intake friendly. 98 is DOHC phase one engine, and has been known to have head gasket problems, and sometimes piston slap caused by the very short piston skirts. 00 and 01 came standard with a rear LSD, while the 98 and 99 had an open diff. 98 have the front bumper with the small driving lights, while the 99-01 all have the big lights. All four year where available as coupes, while 00-01 where also availabe in a sedan, which has the shorter rear spoiler. Personally, I would go with a 00-01, they have the bugs worked out of the engine, are much easier to do intake work on, and have the rear LSD, all bonus's over the 98-99. But I've also heard that if you are planning on going turbo/SC, then a 98-99 might be a better choice, as the MAF is better for FI apps than the MAP. Anyways, that's the important stuff I know, but I'm sure someone else will add onto the list.
*edit* As for the difference between the DOHC vs the SOHC, the SOHC engine actually have a touch higher peak power, but a much broader powerband, while the DOHC has a more peaky powerband. Down low in the RPM range, the SOHC actually has almost twice the tourque of the DOHC. But, once again, if you are planning on going turbo/SC, then the DOHC might be better, since it's easier to play with the intake and exhause valve timing.
